Do not leave your boat in fresh water
either without bottom paint. You will end up grinding hard to get all the mollusks or barnacles
or whatever off, and it will get expensive to fix.
Another problem is that standard gelcoat
is not impermeable to water, water can soak into gelcoat
and get under it, and then into the fiberglass
, and start separating the layers. It starts as small blisters
, can get into large areas of delaminating. It takes a long time, boats that aren't continually in water dry out.
You not only need bottom paint to keep plants and critters from attaching to your boat, you need a barrier coat under the bottom paint to keep water from migrating through the gelcoat. Some boats come with impermeable layers in the gelcoat, check with the manufacturer.