I think if your character(or AI's character) is next to water, they should get the option to willing jump into water at the cost of their own health/energy, since there are situations where you or them would be willing to sacrifice a little to get across some water.
Points in case:
-chests that are only reachable across water (If you happen to lack an engineer, a flier, a cryokineticist, or a thief/assassin)
-AI that don't know how to react if their main path across water is taken out (e.g. the city-defend mission against the Ebon Riders. After I took out the bridge to my immediate north, I could have simply left them there without harassing them (I couldn't resist though), and they wouldn't be able to get across to get to my tent to attack it, leaving me able to take all my troops left and up to Ebon Raban, without any defenders. If the AI had access to be able to swim, then I would have had to stay and deal with them first, or leave a pair of defenders.
-I'm sure there's other cases where this could come in handy too.
If this doesn't make it into the main campaign, I could whip up a mod for the attacks file and characters that adds this ability in and gives it to every character.
I could see the use for this. I have once pushed my ally into the water, so she could then swim to the other side of the stream.