The newer Optimax's are a decent engine, they are pretty good on fuel and comparable to an HPDI. I personally feel they are a slight bit louder from experiences being in them, but they are good engines. The older Merc Optimax's were plagued with problems and had a not so great rep, but the newer gen which your potential year falls into was after the issues were resolved. Only down side to the Optimax is the outrageous price of their spark plugs, Merc couldn't use a plug that had a normal shape and had to flange them different to make their low pressure injection work how they wanted, partially part of the earlier issues, they wanted to be different and complicated and try something new. For an islander, twin 150's is a great hp size, 175's wouldn;t be bad either, anything more is overkill. fuel efficiency should be comparable to four strokes, maybe a a hair better depending on what brand and model your comparing to, but they burn oil sso the end cost is similar or more then a four stroke. I am not a Merc fan persoanlly, but the newer gen Opti's are not bad engines and I did consider them when we made out last purchase when we looked at Whaler's, it would have not been a deal breaker, but was not my first choice from persoanl experience with other merc products in the past.
