The Garmin 12 is good -- it's cheap, easy to use and waterproof. I don't use an aquapac with mine and I've had no problems. If I had to buy another hand held GPS it would be another Garmin 12.
The 76 is the marine version of the Etrex range. It looks cool and groovy, but doesn't appear to have many new features. From a quick look at www.garmin.com
the main differences seem to be:
- larger display
- built in world base map
- more routes (but not more waypoints)
- 2xAA batteries, not 4
- 16 hour battery life vs 24 hour (claimed!)
I don't have prices to hand, but I have seen the Garmin 12 for about £100. I would expectthe newer Garmin 76 to cost substantially more than this, which I would find hard to justify.