[thejeff]

I'm not sure, thematically, why some undead have "poor amphibian" and some don't. Though arguments can be made for some. The behemoth description, for example, says it is controlled by a mage riding it. Maybe he needs to breathe. (That breaks down as well if you look at it too closely.) Regardless, the answer is, as you suspect, standard water-breathing items. (Which also make on sense thematically: How do 50 troops breathe out of a single barrel of air and manage to do anything else - like fight a battle?)

You're right about capacity: 50 men is 100 size units. IIRC, the commander counts towards the limit.

Items grant "poor amphibian". I don't recall the exact numbers, -2 or 3 to att/def/AP? Neifel giants will still slaughter tritons, but may have trouble against more elite forces, especially cold resistant ones. But Neifels usually win by surviving until everyone else freezes anyway, so their stats don't matter much.