Power switch for a dive computer
Don't even try to power it on or off : leave it powered on, but in a low power "sleep" mode from which a button press can wake it by a "pin voltage change" interrupt.

The TI MSP430 processor can "sleep" at under 1 microamp (with a timer running) and wake up this way in a microsecond. Asleep, a small button cell could power it for years so there is nothing to be gained from power down. If it doesn't have enough processing power, there are ARM based MPUs which aren't far behind in deep sleep power or wakeup time. This is a $10 starter kit for a couple of the smaller MSP430 devices.

Magnetic sensors (reed switches) are the way to go for button press detection. They need no power while asleep. Hall effect switches may work, though I don't know how low you can get their power consumption. Or possibly opto (reflective) sensors as Anindo suggests, though they consume power all the time. 

Capacitive sensors may have difficulty detecting fingers when underwater! However a capacitive sensor is probably ideal to signal immersion in water.