Shack...there is nothing that needs marinizing on a typical TV and a small inverter can let you run off 12Volts. What you should be concerned about is that the TV you buy is HDTV capable because in about 18 months, all regular broadcast signals will cease and "normal" older TV's will need a converter to tune in the new HD channels. There are lots of NON HD bargans out there still but as of 3/1 this year, all new TV's had to have built in HD tuners. Nothing wrong with getting a "bargain brand" LCD set but make sure it has an industry standard plate mounting pattern so that whatever you buy to mount it (we used a swing arm) will fit.
Finally...look for HDMI connections (at least two) as this will simplify hookups on the boat and is the future of connectivity with other devices like Amplifiers, DVDS etc.
You don't say what size you are looking for so it is hard to b specific with recommendations. Also...will you be hooking into an audio system (surround sound) or just using the TV's speakers?
Sander...as long as you have a good bulkhead there is no problem. Many boats and motor homes are mounting this size set today. In most boats you will need a tilt mount to tilt the screen down a bit for proper viewing. I would through bolt with backing plate since you are dealing with about 50 pounds. Check out this article for an example:
Note that such size sets typically use about 250 watts or 20AMP/hours per hour of viewing so you may need more than a mount to run one!