The survivors of the river battle run out of luck…
Here are the rules that I came up with to handle capsizing, swimming, and drowning (mainly drowning);
Boats: Capacity=5 models, speed=3 (hands and sticks), capsize on a d6 roll of 7+
Boats can capsize and dump their occupants into the water if their capacity is exceeded or if the occupants become too unruly. A stationary boat, one that did not activate, does not need to test for capsizing. Capsizing tests occur after movement or any form of melee has occurred on board the boat. The capsizing roll is modified as follows;
+1 for each model that exceeds capacity.
+1 if melee combat occurred on board the boat this turn.
If a boat capsizes then all of it’s occupants are dumped in the water, the owning player must roll above the model’s DEF score to avoid drowning. Suppressed units that end up in the water have a -1 to their avoiding-drowning roll. If a model survives getting dunked they may swim at a speed of 1 or just get back on the boat.
Models on boats count as having soft cover (-1) to ranged attacks that originate from off the boat.