i'd say rip apart an old toughbook (some had waterproof touchscreens) and/or use a button press interface rather than a full out touchscreen and what not.
For headphones, E2C's

. If you can hear the outside world, you installed them wrong

. hehe, no, but still, they're really good and high quality stuff. However, if you fly off...
Two options i would suggest:
Better and more expensive:
If you ride hard, a good idea would be to use an SBC (single board computer). They'll run full pentuims and what not and everything woudl be okay. Use a compact flash card as yoru HDD (a 4-8gb will do the trick). Sure, loading will be a but longer, but what you lose in loading, you'll gain in rugged reliability. Waterseal a USB 2.0 enclosure for external digs or even use a USB 2.0 card reader with a second CF card for media. I mean, i doubt you'll be watching Deep Blue Sea while out on the boat, but other than iguidance (love to see you plan a route on this thing), music, and SOME video, how many hours are you going to spend on this thing serious? Just take the tunes and vids you want with you on the boat, and change things around when you're done.
Less expensive, still pretty good option:
Newer Pocket PC. Touch screen, decent spead, plays what you wnat, when you want it, and short of PC gaming, it's everything you need in a small package that you can mold and waterproof with ease (use a hiighquality screen protector for the glass por just cover the whole thing with a waterproof film. It should still register the taps). Include a 4-8gb CF card. The best part is on the way to the beach, you can transfer the files you want via activesync using wifi, BT, or a USB cable. Just over the USB port when you're done

. If you get a good one, and a bluetooth headset or something, you can use voice control to do a good job cycling through your media. If you want, at some points, using TSC or VNC, you can actually have and control your whole desktop on your PPC screen from your carputer if you're in Wfi range, or map your computer's HDD to it, so if you forget to send over a song or have a vid you want to stream

...