underwater troops

[Arryn]

Humans are size 2, while Jotuns (giants) are size 4, thus only half the stated number of Jotuns could enter water (assuming that's your problem, since you have not said what *type* of troop you're having problems with). As for mounted humans (knights, cavalry), they are size 3. So 25 * 2/3 = 16, which might explain what you're seeing.

[Ironhawk]

Yeah - the waterbreathing ability for additional units is listed as "X units". But in fact it is "X size 2 units worth of space", which you can fill up any way you want.

[Vynd]

Several of the waterbreathing items actually mention in their descriptions that they can take fewer giants or cavalry underwater than they can infantry.