Kennya & all,
I am assuming you live in Oklahoma and may even keep your boat on Texoma?
Well, I would be very surprised if 1300 BTU is going to cool your boat. Maybe you just have a very small boat? I don' know. I have a 16000 & a 12000. The 16 really is almost enough... excpet on the hottest & coldest days (it is harder to heat then cool on a reverse cycle).
As far as the top mount unit, You might save a few bucks and try what I saw another guy try. Ok, now, this is reall-real-real red neck engineering, but:
THer is a guy on another dock that took a large home mount unit, built a wooden box around it for weather, left open the exhaust (hot side) side and ran house style ducting into the hatch from the cooling vents. The unit sits on the dock.
I am not neccessaril reccommending this, per se. I have actually thought about sending it into Americas Funniest Home Videos, but am afraid of death threats from the LCRN (Local Chapter of Red Necks). Still, it is a cheap and easy way to do it.
If you do want to go with a small water cooled unit, Mermaid makes one in very small BTU's and they are supposed to be easy to install. They look like they would be. Still, as has been pointed out, you will need a genny to run it away from dock. At dock, it is no problem, of course.
PS The water cooled units work very well, even in the tropics... I can tell you. It is not how cold the unit get that I have found is the limiting factor, it is how "hot" the deck is and how much of that is radiating down with full sun.