Not really the easiest thing to accomplish right now out of the box.
There are swimming animations in the solder characters, but not the player or the third person wizard yet. That would be obstacle one.
There is not any water physics right now either. so a script will need to be made for that to float the boat.
We have a few work arounds in the scripting thread in smallg posting, index to his scripts are in the first post.
Maybe chaining some of these together can get you a decent illusion of what you what till the water gets reworked for an update.
Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz, 2400 Mhz, 4 Core(s), 4 Logical Processor(s), 8gb RAM, Nvidia gtx660, Windows 7 Pro 64bit