Leaking bellows are one of the most common reasons inboard/outboard (I/O) boats sink at the dock. Even if they looked OK in the fall, carefully inspect them now before your boat goes in the water. Check your bellows with the sterndrive raised and lowered, looking for cracks or sea life such as barnacles and oysters that are sharp enough to tear it open. Many I/O boats also have exhaust and shift cable bellows, so check them as well. If one bellows is bad, they should all be replaced.
Over winter, your cockpit and deck drains may have accumulated a lot of crud, from leaves to dirt to critters. A clogged or slow cockpit drain can sink a boat, especially an older one that might already be squatting lower in the water. Send a high-pressure burst of water from a hose into each drain to make sure it runs free. While you're checking how fast it drains, inspect the thru-hull fitting as well. Plastic fittings degrade in the sun, and if they crack, they can allow water back into the boat.
Hoses And Thru-Hulls
Now is the time to check every one of them before your boat goes in the water. Every fitting below the maximum heeled waterline must have a seacock that can be closed. Over time, seacocks get stiff and may not be able to be closed when you really need them — like when water is gushing into the boat from a split hose. Make sure every seacock opens and closes freely — now's the time to address the stiff ones, before the boat is launched. While you're inspecting the thru-hulls, take a look at the hoses and clamps as well. In fact, do more than take a look — give them a firm, twisting tug and make sure the hoses and clamps aren't past their prime