Can one . .

[HIC SVNT LEONES]

Set units with the ability to swim? I've not actually tried yet, but is that possible?

[Roshak]

In BI it was possible but I think in this game you cannot. Apparently the knowledge of swimming was lost with everything else

[Space Wolves]

No, perhaps a mod, but it might be to difficult.

The Medieval Soldiers like Roshak said, seem to have lost the abilities to swim..

Perhaps An evolutionary gap?

[rainkloud]

In BI it was possible but I think in this game you cannot. Apparently the knowledge of swimming was lost with everything else

Hahaha The plague must have hit only those with swimming ability or maybe no one wanted to sign up for Barbarosa's swim academy.

[Grand Duke Vytautas]

Hahaha The plague must have hit only those with swimming ability or maybe no one wanted to sign up for Barbarosa's swim academy.

lol +rep for that (edit: seems I can't give atm, will do when I can, any more rep traders here? )
Barbarosa's swim academy - hilarious

[MattErickson]

Oh perhaps all those pounds of armor. Maybe if they were peasant archers or a unit with little to no armor. Or it was a shallow ravine and they could cross on horseback. But again, lightly armored scouts or horse archers.

May be possible with a bit of editing and modding.