Great question -
A lot of the features you want are available on a lot of GPS units, but first let me say, I am biased. I have used Lowrance, Magellan and Garmin, and I prefer any Garmin product over 80% of GPS units out there. Depending on what you want to spend, you could go from around 150 dollars all the way up.
The one tricky part is depth.
Based on what you are saying, I would recommend the Garmin 60cs or 60csx.
They both have digital compasses, they will offer trip computers to tell speed, and they will give you a top down view, though not necessarily all at the same time.
To get depth information, you would have to get the Blue Chart software which utilizes depth soundings on the map display, but I think that would give you a good idea of how deep you are.
The added benefit of the 60csx is a built in barometer which you could use to do some rudimentary weather forecasting too.
Take a look at that, if the cost is steep, the 60 series do have some variability and you can definitely add or subtract what you want.
-Erik