Two boat policies can name the same vessel and settle a total loss very differently. The clause that decides it sits near the top of the declarations page and takes about ten seconds to read once you know it is there.
The difference
An agreed value policy fixes the figure when the policy is written. If the boat is a total loss, that is what is paid, without argument about depreciation. An actual cash value policy pays market value at the moment of the loss, which on a hull with years behind it is a considerably smaller number and a considerably longer conversation.
Agreed value costs more. On a newer boat that premium difference is often not worth it, because market value and agreed value are close together anyway. On an older hull they diverge steadily, and that is precisely where the cheaper policy quietly stops being good value.
Where it bites hardest
After a named storm, when a great many boats are damaged at once and market comparables are scarce and contested. An agreed value figure removes that argument entirely, which is worth something beyond the dollars in a month when you have other things to deal with.
The related question is what the policy does with a partial loss on an older boat, because some carriers depreciate specific components regardless of the settlement basis on the hull. Canvas, upholstery, electronics and outboards are the usual list. Bring the declarations page to any office and we will tell you which basis you hold and what the partial-loss language actually says.