I have used a Garmin eTrex GPS for two years. It provides the basic fuctions a person needs to do navigation and can be nabbed for $90. It is water resistant, but I use a small cell phone dry bag for $5 from a local dive store.

Before the eTrex I had a Magellean 540. It was an older model and never worked well. The eTrex is great for the price.