There are many choices out there, best bet is to go to your local outdoor store REI or something and talking to their sales people so you can see side by side comparisons.
I personally use Garmin Vista HCx, it's waterproof, shock proof and allows me to upload topo maps on it. Then you want to go to GPS Waypoints
and download the waypoints for the rivers you need. It will help you with the river maps.
I only use my GPS to show where I am at and use the river maps for the navigation purposes.
PS: I second DanOrion on DVD rather than MicroSD. That's exactly the route I took.