A boat or camper, like any other form of abode, will collect clutter if you don't make a conscious effort to avoid it. It is just about as easy to accumulate clutter in a boat, an RV or camper as it is at home. Fortunately, at least for us, there are usually fewer opportunities to collect unnecessary souveniers out on the water than in campgrounds or while traveling, but it is still way to easy to bring aboard items for a single outing that seem to never find their way back home. And, while it may not be as likely to accumulate a lot of unnecessary stuff on a boat, it is likely to be more important to get rid of excess weight. Excess weight in an RV can impact gasoline mileage if there is enough of it and, if heavy enough may affect the ride, performance, and safety. On a boat, any excess weight is going to affect displacement -- how deep your boat sets in the water and that will definitely affect handling, performance, comfort, and safety much more than it does in an RV or tow vehicle.
Just like an RV or a set of camping bins, boats will benefit greatly from routine inventory and organization. Often you may find it even more important to be sure you have everything you need on board and aren't carrying around a lot of extra weight. Extra weight usually means a drop in performance for both power boats and sail boats and can affect handling and safety too. Because there aren't as many places you can acquire more "stuff" out on the lake or the ocean, you might not accumulate excess as quickly as you would in an RV or even when tent camping, but it is way too easy to "bring along" something for a specific outing and leave in on board indefinitely. Over time that can amount to a lot of extra weight and a lot of stuff you have to move or sort through when you need something. A lot of things that seemed like a good idea at the time may turn out not to be necessities and might better be left on shore. Every time you launch your boat and every time you have downtime at anchor is a good opportunity to check your inventory. You gain at least three benefits from this: 1) identifying items that are used up, damaged or expired so you can replace them. 2) finding things you don't need on board all the time, and 3) locating things you need but might have forgotten where they were stowed.
There are very likely at least as many ways (or more) to organize your boat as there are sailors but there are some general guidelines that can make life easier. Unless you have a really big yacht, space is going to be at a premium and you will want to make use of every bit you have and do it efficiently. Some things, like cooking utensils and food related supplies obviously belong in the galley. Things like boat hooks should be stored somewhere topside so you don't have to dig them out when docking. Tools should be easily accessible. My Venture 24 sailboat has a small compartment right where you step down through the companionway into the cabin that I found to be a good spot for my tools and spare parts. They are out of the way of normal activity but quick and easy to get to when they are needed. Things like life jackets and other flotation devices should always be within easy reach. You want to be able to don your life jacket at the first sign of turbulent conditions and throwable flotation devices (used to aid passengers who fall overboard without a life jacket) must be immediately available. Coast Guard regulations require you to have a life jacket on board for every passenger and it is not a bad idea for everyone to wear one at all times. If you choose not to wear it all the time, ALWAYS make sure it is within easy reach and you know where it is. Throwable floation devices, like life preservers and throwable cushions should be readily accessible from the cockpit. Many boats have storage under the cockpit, accessible through one or more hatches in the cockpit. Throwable cushions are a good way to add both comfort and safety to the cockpit. This is also a good place for other things you might need while underway. Some things need to be right at hand, like the winch handle for sailboats. When the boat is docked in port or the marina you might want to stow the winch handle inside to avoid it being stolen, but when sailing it is good to have it in a holster attached to the cabin bulkhead next to the companionway hatch. within easy reach of the winches.
Tool kits are essential to maintain your boat and effect emergency repairs out on the water. However, you should try to limit your tool kit to just what you might need -- and know how to use -- to get back to the dock -- any dock -- if something goes wrong. I have seen articles that suggest having cordless drills on board, but personally I think that, unless you have a large, powerful boat, they are likely to add too much weight and take up too much room for the few times you will actually find your need them. And, most of us don't get out on our boats as often was we would like and the batteries are likely to be dead by the time we need to use the drill/driver. I would recommend sticking with a basic set of hand tools that will let you take care of most routine tasks and basic emergency repairs. Such a tool kit doesn't have to take up a lot of room or add a lot of weight. And, as always, look for multi-use tools that can help minimize what you need to bring along. Some basics to consider include pliers, wrenches, screwdrivers and and adjustable wrench like a Crescent wrench. Check to see if your boat has any fasteners with special heads that might require square drives or torx bits. I have seen recommendations to carry cordless tools like drill/drivers on your boat. I carried one on my sailboat for a while but found I didn't really use it often enough to warrant to extra space and weight it took. I left if out when we switched boats. I think I might carry little Worx 4.5 volt cordless screwdriver though. They take up little room, don't weigh much, and are surprisingly powerful!
After you've had your boat for a while it is a good idea to take time to go through and take inventory of everything you have on board. Over time some needed tools or supplies will get lost or used up or become outdated and need to be replaced and you will tend to accumulate miscellaneous items you don't need on every trip. Taking time to take inventory lets you get rid of unnecessary stuff, replace important missing or damaged items, and, in general, refresh your memory of what you have and where it is. If you find stuff you never use, seriously consider taking it home unless it is a critical tool or survival item. When you find damaged or missing tools, replaced them. Outdated supplies should be discarded and outdated or used up supplies replaced. It might also be a good time to consider how effective your current organization plan is and to relocate frequently used items to where they will be more easily accessed. However, because we are creatures of habit there is sometimes great value in keeping things where "they've always been" rather than moving them to "where they should be". It is also more important to be sure you have essential items and can find them quickly. Emergencies on the water are usually unforgiving.
Deck and Cockpit clutter is particularly dangerous. When you are underway you need to be able to move about the deck or cockpit safely and have ready access to all instruments and controls. Clutter on the deck or in the cockpit can seriously impact your ability to safely handle your boat and keep your passengers safe. It is all to easy to let sheets and halyards on a sailboat pile up and they become trip hazards or get tangled so you can't use them when needed to adjust the sails when the wind changes. Likewise, dock lines on all boats need to be carefully and safely stowed, both on the boat and on the dock.
There are SO many nice gadgets and goodies for boats it is really easy to accumulate a lot of seemingly worthwhile items. However, if you don't find them useful or don't use them often, you might be better off tucking them away in the garage until you are ready to use them. Otherwise you may find your boat cluttered with "good"things. Space on boats tends to be even more precious and limited than in RVs so you REALLY need to make the most of by getting rid of unused or unnecessary things.
Boats can also accumulate unwanted stuff on the hull below the waterline. A buildup of barnacles, algae, and/or other materials will add weight and resistance. Stuff might build up on the hull, rudder, and propeller. You would be surprised how much drag even a coating of algae can create. Boat bottoms require special paint. Usually it contains copper to reduce growth of barnacles and is designed so ablation helps keep junk from adhering. While that helps keep the bottom clean, it does mean the bottom must be repainted on a regular schedule to maintain protection, performance, and appearance. The same applies to rudder, propellers, center boards, and any other equipment that resides below the surface. Consider the sheer square footage of surface area of the hull underwater and what happens when the slick, freshly painted surface gets covered with algae, barnacles, or other debris. Even a small percentage increase in the coefficient of friction when multiplied by the surface area is going to have a big impact on performance. Even if you are not racing you want your boat to move through the water as smoothly and easily as possible.
Clean it up!