If you already have a home theater setup mouldering away in a box, I'd say go with the cheap CPU approach. Although, I think the question really is whether you intend to use the theater while you're out cruising, or whether it's more a dock-side attraction. If it's the latter, I wouldn't worry about the amperage or dc vs. inverter question, since it sounds like any of your CPU or PS3 based solutions would draw a substantial amount of power anyway and you might as well just plug them into the dock.
If you intend to watch/play while out on the water though: 1) Are you nuts? You're in the wild blue yonder!
; and 2) I'd take the laptop route since it solves the DC problem for you and you could still game on it if you get one with a dedicated graphics card. Plus it's easier to stow.
For my 30'er, I actually got rid of an AC/DC tv/dvd player since it seemed too old fashioned for my purposes. Now I'm rocking an older dual-booted Imac with the stand cut off and mounted in the v-berth, so that I may blow my enemies into tiny bits (TF2 ftw) or curl up with Hulu while at the dock or via inverter in a pinch. I also picked up a netbook to do my casual work and surfing from the salon table, or to act as my chart plotter when I hook it up to the GPS. It works for me, but you may have grander ambitions. Good luck either way and let us know if you devise the perfect setup.