I got one that was a little thicker than your average one but it floats as I also use it in the boat.
The limited features that I can think of that you may like to consider include:
- Flashing LED lights so searchers might spot you more easily
- Floating ability
- Floating with aerial upright (they don't all do this !!)
- Carry case
- Auto or manual activation (usually water and not shock)
- Long lanyard (generally for boat use)