The second year mini pets and several of the special mini pets seem to be unusable in- and underwater.
If summoned on land they vanish as soon as you enter water and start to swim or dive and you cant summon them if you already are in water. They also dont automatic resummon if you leave water.
I can understand that some minis dont have underwater motions , but this happens to the human, norn and asura minis, as well as to the minis that share the skin with some of first year minis.
For example first years Hylek Nahualli and Yaotl the Fierce can swim, but neither of the second year Hyleks can. First years Skale Blighter swims, second years Skale Brackish doesnt. Dredge Excavator swims, Resonator and Strazar dont.