The Kraken:The company I mentioned has solved the problem. They make water tight cases for the most popular brands of hand held GPS's and a long antenna lead that leads up to the external antenna left at the surface in a float. There is some type of amplifier (don't know what type, I'm extremely ignorant when it comes to matters of electronics) the boosts the signal down to the receiver. If I remember correctly, the current max depth for the antenna is only about 25 feet or so.
This would be fairly easy to do as a DIY project. There are plenty of external antennas for current GPS's . The antennas are already waterproof..or at least rain proof, so mounting them on a float at the surface is no problem. Don't remember off hand how long the one I installed on my boat was but seems like it was around 25 ft. A waterproof cable and a waterproof box for the GPS would be the challange. As for me, my compass works fine.