The one thing that I wish someone would make is a life jacket. I'm sick of bailing out over the water and drowning.
I know that Colonel Klink is working on one to be released in his next Fishing Boats release, however, I still can't believe that this game has been out four years and still nobody has made one!
If I was any good at making add-ons, I would undertake it myself, however, I know very little about it. Speaking from a logical/programming mind though, I know what the best way to do it would be (IMHO):
1. Make it an item that adds-on similar to the NVG's or Binocs.
2. When worn, it detects whether you're over water or land. When over land, it's just a visual model. When over water, create a transparent box underneath your person so he/she can stand on it with head above water. This *should* allow for movement in the water simulating treading water without drowning (similar to what happens when you wade in the edge).