I've searched around and found what I hope to be a perfect solution. It's called a Bullet M2 HP, made by Ubiquiti Networks. I don't know exactly what it is, but if you give it an antenna it can pick up WiFi for miles, apparently.
So, around a hundred bucks later and a week later, I have one of those Bullets, something called a PoE injector, and a foot-long omnidirectional all-weather antenna. Let's go.
Maybe thirty minutes and a beer later, I've got the antenna attached to the Bullet, the Bullet connected via ethernet to the PoE injector, which is connected to both my laptop and a 12 volt power drop I scavenged from the old chart table lamp (it was blocked by my gps chartplotter anyway). Go internet!
Or not. I thought I would be all set. I didn't electrocute myself, everything looked right, and I had followed the [sparse] instructions that were printed on the bullet's box..
A little network troubleshooting (something to do with DHCP) and several beers later, voila! All the free internet porn I can ever watch. Now I just need to con one of my buddies into climbing the mast to mount it optimally.