| What AP's can operate on 12 Volts?

The Linksys WRT54G series can. I've got a pair of them on my boat. One's
in the radar arch maintaining the link to shore. The other's down in the
cabin providing an on-boat wifi network. Works pretty well once we're at
anchor.

| Can I do this with just a AP, wired to the lan jack on my laptop?

You could do it with a wire straight from the wifi unit. But having a wired
laptop might not be terribly convenient. You'd have to deal with that cord
all the time. Run the wire from the up-mast device to the in-cabin device.

Remember when considering a high gain (especially a directional high gain)
antenna that boats rock and even if in an small inland lake you can get
considerable signal drop out if the gain is much over 3db.

True, and higher gain may also pull in a wider set of networks. You'd end
up listening to "too many" and get even worse performance. And the
radiation pattern from most of these antannae is shaped like a donut. When
the boat is rocking that donut gets tilted up and down, in and out of the
coverage area. Higher gain antennae have a much narrower area of coverage.
So any rocking with a typical 14db omni would render it almost useless.
Same deal goes for a directional antenna, if you swing on anchor there goes
the coverage.

Depending on where you're planning on travelling and how you're anchoring
there would be different recommedations. I've had a 10db omnidirectional
stick on my radar arch and it's done a pretty good job of pulling in wifi
signals.

-Bill Kearney