I'm not so surprised, or upset, over decreases in the "agreed value." I only ever assumed that the "agreement" was made at the initiation of coverage. At that time, the insurer usually asks for a survey, and the owner squabbles if he thinks the boat is worth more. Then an agreed value is reached.
After coverage is initiated, the VAST majority of boats will depreciate significantly over time as age takes it's toll. Insurance is a probability game so the insurer is correct that, by-and-large, boat values decrease yearly.
If you're one of the exceptions-to-the-rule, as many here are, who has improved his boat over time, the insurance company has no way of knowing this until you formally notify them. I don't see that as an undue burden since, again, its an exception to the general rule. It seems reasonable that an insurer would then ask for proof of improvements, or perhaps even a new survey before making an upward value revision against their preset depreciation assumptions.
Insurance is a for-profit business, not a social program, I'd prefer that my insurer lowers most other peoples "agreed values" regularly (that's just sound business) and I'll worry about increasing my agreed value after I've made substantial improvements.