The summary basically sais it all. On my survival world (first created in 1.5.0) I get skeletons and zombie villagers spawning under water. I've found several skeletons at the see bottom floor, far away from any land, and found zombie villagers in totally enclosed fully watered up caves.

Also, if you completely emerge a skeleton in water (say a 5x1 water column with a skeleton at the bottom) they won't drown or try to swim up. They'll happily stay there forever. Same for zombie villagers.

Zombies will ofcourse turn into drowned. AND spawn the trident, which they shouldn't under those conditions.

In general, spawning is fully borked on MCPE. I have an areay of 18x18 chuncks and all cleared out of mobs (using mcpe_viz to double check) both above ground as below, and if I build a platform there, nothing will spawn, as I explored a huge area of the map, and mobs don't despawn, so the 200 limit is reached.