It's my understanding that:

Generally, you're covered to the limits of your coverage, for most causes INCLUDING stupidity. So it's an adjustment between insurance companies. If you are found to be intentionally trying to damage your boat or commit suicide or a crime, you probably would not be covered.

In the case of a crash where you exceed your coverage, then you may have an issue. Say you cause a $100 Million yacht sink and the insurance company decides to go after you for more than your coverage. Personal injury claims might become an issue which exceed limits or move outside the coverage descriptions of your marine policy.

In some states the apportionment of risk is strictly proportionate. In others, I believe, it may not be strictly proportionate. Usually it's a jury or judge that ascribe relative liability; however, they may be informed by the finding by the coast guard or NTSB or whatever.