Stosh, Not going to get into a agm vs lead acid debate but i have two optimas so i can give my actual experience. I use a optima deep cell for my house battery and a optima starting battery for my engine which is a 200 hp. Main reason i tried these as i wanted to free up some space in the back and i moved one battery into a compartment under my front seat and the optimas are sealed agm batteries so they do not leak any fluid and can be placed in any position. A lead acid battery would have leaked up there and made a mess so i wanted to try the optimas. Im into the 4th season and i have had no issues so far. Like any batteries you need to keep them charged when sitting for extended period and you should have a smart charger that handles agm batteries as they do not like the higher voltage charges of a lead acid would get.
There are lots of quality lead acid batteries for 1/2 the price and a lot more MCA and may be the better choice in a lot of situations but my main reason for the optimas was they are sealed and clean, can take more of a pounding on the water and they don't leak battery acid out.
good luck