Ideally, you would want to get it down to bare aluminum. If there is a lot of paint, you can use a product called "Aircraft Coating Remover" (oddly, a warning label on the back of this stuff says "do not use on aircraft" LMAO) Similar paint strippers can be bought by the quart, or even gallon, at a home improvement store.
WARNING: if you use this stuff, wear full length rubber gloves, respirator, and safety glasses, and have a water hose nearby!! It contains methylene chloride, as well as some other caustic chemicals, and if it gets on the skin, it feels like your skin is on fire! Be very cautious with it, and use it in a well, ventilated area.
As it works by the process of its fumes, and its evaporation, you should strip small areas at a time, instead of applying stripper to the entire boat at once. Trying to do too large of an area wastes product, and exposes you to a lot more fumes. Work small areas, applying the product with a brush, letting it blister the paint, then scraping with a putty knife to remove loose paint.
If done properly, this stuff will remove multiple layers of paint, all the way to bare metal. You won't have to do much sanding at all.
Again, once you get it to bare metal, you need to use the phosphoric acid etcher (also called "Ospho") to etch the surface of the aluminum....and to remove excess aluminum oxide (as this is the primary ingredient in sandpaper, as well as the reason why paint will not adhere properly to bare aluminum)
Rinse thoroughly, and once it is dry, apply the primer (zinc chromate) and then, apply your coating, be it epoxy, undercoat, or truck bed liner.
If you follow these steps, you'll have a durable finish, for sure.
And one more thing, if you are using your boat in salt water, for heaven's sake, put a zinc anode on it!! I cannot tell you how many boats have come to my welding shop with corrosion and leaks that could have been prevented by proper galvanic protection.
I forget the MILSPEC rating, but basically, a 16 foot boat with a 4 foot beam in the water, needs about 36 square inches of zinc for proper protection.
4 steps to minimize problems with corrosion, and I tell this to all my customers with boats:
1. Fabricate and install some type of mounting bracket below the waterline, on your transom, and mount a few zincs.
2. Keep the coating of paint below the waterline intact, keep a layer of copper thiocyanate (NEVER copper oxide) paint on all surfaces below the waterline
3. Inspect and replace zincs once they are at 75% of their original size (keep a new one on hand for comparison)
4. NEVER leave fishing sinkers, steel waterfowl hunting shot, loose ammunition or shell casings, screws, or any other loose metal items in your bilge. This is a guaranteed way to eat holes from the inside, because you are setting up galvanic action, with your boat being the less noble metal. (You CAN put a piece of zinc, or magnesium in your bilge, though, and this will prevent galvanic action caused by fasteners, thru-hull fittings, etc.)
