In article ,
B A R R Y wrote:

Many newer marine units have external water temperature, depth sensor,
and water speed sensor capabilities, as well as the same memory
capabilites as aviation units. While airplanes need aviation waypoints
and related information, boaters need serious waterway chart storage, so
memory is just as important. The better marine units have the
waterborne equivalent of our VFR charts stored and displayed by the
unit. I really think price differences have more to do with our $70
inner tubes and $50 carriage bolts than technology differences.

Marine units (at least the high-end ones) often have tide charts built into
them too. Unlike winds aloft, tide heights and tidal currents are very
predictable and can be calculated years in advance. A good GPS unit will
have these tables built in, and can superimpose on the displayed chart a
field of current vectors, and tidal water depths.

Marine units also have a feature which is very handy on a boat, but more or
less useless on an airplane (unless you're hauling skydivers). Hit the MOB
(Man Over Board) button, and the unit will immediately store your current
position as a user waypoint and start navigating to it.

They also often have an anchor watch feature, which sounds an alarm if you
ever get more than N feet away from your current position. You do then
after you've anchored to alert you to the possibility that your anchor is
dragging (you set N to however many feet of rode you've put out).

Oh, yeah, and marine units tend to be waterproof. I don't know of many
aviation units that are designed to 1) float, and 2) survive be submersed
in salt water.