Yeah, the agreed value is good, but only counts towards the whole boat (and trailer if on that at the time) being written off/stolen. If the engine alone is stolen (for example) then you get market value, less loss adjustment for the age of the engine, etc. I checked with them when renewing mine this year. They did tell me that the assessor would look at the engine's value but also take into account the cost of renewing like for like.
In short - if someone steals your engine, make your boat disappear too!
Porthcawl however were the only company that would offer agreed value for my boat, with it being 2003. HKJ, for example, wouldn't as it's too old for their policy.