[Wauthan]

Units accompaning a watermage under water all get a hidden "poor amphibian" ability. This is also true for any items that gives waterbreath. The penalty is a simulation of the unusual nature of fighting under water. Try moving around fast while swinging a heavy stick when you go snorkling the next time. Imagine doing that in full armour and on a horse.

Underwater units get no penalty for fighting on land. The assumption is that it is easier to adapt to fighting on dry land if you're a sealubber.