Basically, when you're pulling your boat to and from the lake it's covered by your auto policy, which is good news - the bad news is that it is covered only by your auto policy which may have very limited coverage as compared to your boat policy for loss of life, bodily injury or property damage.
When it's parked on your property your homeowners policy may provide some coverage if it is stolen or damaged but probably won't cover things stolen out of the boat or vandalism.
The best way to provide additional coverage on a boat out of water (and probably a boat in the water as well) is to get an umbrella policy - these are pretty cheap - mine is less than $200/year - and come on top of your home owners or auto policy when those limits are reached.
