3x10 key keyboard help

OK, I am building a 30 key (3 columns x 10 rows) keyboard, to be labeled in qwerty fashion with a space and 3 function keys. The goal of this project is to have a qwerty keyboard that is about 3x5 inches and a screen above the keyboard, with an arduino mounted under it, shield fashion. As I type text, it will be sent over serial to a similar unit. I will have one, in a sealed case, filled with insulative oil, Velcro-ed to my diving suit. The other, connected by a 4 wire tether (power, and tx/rx) or ultra-low frequency serial link (I'd really prefer the wireless ;)). I'd like to know what circuitry (multiplexing, charlieplexing, ect.) to use to attach the keyboard to arduino, since simple high/low logic won't work (not enough digital inputs) and I'd like to use some of the I/O's for a depth/pressure sensor or O2 sensor. If I do use a 4 wire tether, I assume a 5v serial signal won't make it past 100 ft reliably, so If anyone has an idea on a serial repeater to boost signal strength, that would be really helpful. Schematics would be nice. Just figure I'd post this and see what help I get, so since it's kinda a crap shoot, just post away ;D Also, I am aware that there are very nice commercial systems like this, but they cost much more than my budget (around 75 to 100 bucks) and you don't get the fun of building it yourself. Or the bragging rights :D

Thanks in advance, /me

Sounds like a cool project. I just did some thing similar.

Have a look at this: http://www.arduino.cc/playground/Main/KeypadTutorial You set up a matrix. In your case 10X3 could be done with 13 pins. You could get it down to 11 if you wire it as a 5x6 (keys can be any lay out.)

This display needs just 1 output. And you need another couple of pins for the link. A nano has 14 digital and 8 analog IO. Should be plenty for your needs.

A max485 rs485 chip could work for sending data over wire. I would use water proof and flexible cat5 (the rugged stuff with stranded conductors.

I have no idea on sending data via sound under water. Sounds like a fun thing to try though.