Tested on Samsung Galaxy S6 (SM-G920W8), Android 6.0.1

I could not drown any mob (other than blaze, magma cube which are hurt by water).

I tested trapping mobs in water under blocks, trapping them inside a tower of flowing water and spawning mobs deep under water so they should die before floating to the surface. In none of these scenarios did the mobs die. I tested sheep, pigs, chicken, spiders, zombie pigmen and ghasts in a mix of creative and survival single player.

Squid spawned on land die as expected.