Hi Charlie!
I would say to leave it at 0x650c: there are other items in the 0x65-- series that are not obviously made of water (e.g. sand bar), but which are located in or near water. 0x650c is listed as "island" by cGPSmapper, but falls in the "water based" category in Mapsource / GPS. I guess it kind of makes sense as an island is in the water.
Thank you for your answer. A little bit I thought that my misunderstanding came from a ambiguous translation in the Garmin software.
0x660x is listed by cGPSmapper as "gut" and is adjacent to other land features like gap, isthmus etc so I assume it means either some sort of cut-away in the land by the coast, or a promontory into the sea. As a native English speaker I can't say I've ever heard the term used in this way, but perhaps it's a specific geographical usage.
Or it is an american term that is uncommon by the British? ;-) After what I have found in the German menu of my Oregon I think everything is possible ... Dani