Co Insurance
Naming another party as coinsured on your policy protects them from being liable in an action taken against you. For example, if someone has an injury on your boat while in the marina, the injured party may try to also claim damages against the marina, assuming they have much "deeper pockets" than you. By being named as coinsured on your policy, your insurance would come into play before theirs. There are other possible scenarios, but the naming of marinas as coinsureds is indeed very common.