I think this is a classic example of why a GM is needed.
There are many variables.
Is the water flowing like a river and likely to carry you away, of still like a pond?
Is the water deep enough to cover your head?
Are the models close enough to the bank/shore to make a mad dash to get out just in time or are they in the middle.
Are the models ogres, halflings, men or undead.
Undead don't care about drowning, so unless carried away by flowing water they will be fine?
So many things to make a blanket ruling unfair/unrealistic.
I think my rough base for tests:
For a river that does not cover the heads.
Each model caught must roll under their STR on a d10 or be lost.
Ogres and trolls add +1, giants do not need to roll.
If you pass, move at 25% of normal. Must roll each turn though to keep beating the current.
For water that does cover the heads.
Tough poop, they are lost.
You should know better, you deserve it.
For a lake/pond that does not cover the heads.
Each model caught must roll under their STR on a d10 or be lost.
Ogres and trolls add +1, giants do not need to roll.
If you pass, move at 25% of normal.
*Undead treat all tests as not covering their heads.
I think this will be my starting ruling, I will ponder upon it more.