From what I remember reading, people were having a lot of issues with the Rivermate...the majority of which were with the sheath. With the original sheath, people had trouble with the knife falling out. Then they redesigned it, and people had trouble getting the knife out one handed. I think they have since discontinued the design, as I no longer see it listed on their website.

The River Shorty, on the other hand, has been around for a long time and it well proven. However, it isn't really a great design for use on land, as it's pretty much an entirely serrated blade and the handle design is made more for clamming/prying than extended cutting.

Benchmade has a nice looking dive knife out now, the 110H20, however it goes for around $85, which I'm guessing is a bit more than you might be willing to spend based on the options you listed.