If lightning strikes at sea, a skeleton horse is spawned. If lightning strikes that horse, four skeleton riders appear (usually in armour) but one reliably sinks to the ocean floor and sits there without suffocating. It's hard to reproduce ⚠️ but I've seen it several times.
MCPE-19167
one of four skeleton horse skeleton riders sinks to ocean floor
Resolved
Cannot Reproduce
0
(Unassigned)
Unconfirmed
Tablet - iOS - iPad Pro
iOS 10.1.1
skeleton, skeleton_horse
0.16.2
Retrieved
I'm not really sure how to resolve this ticket... I can't repro the part where the skeleton on the skeleton horse doesn't drown when submerged. But I also suspect the behavior where only one would sink was because the other skeletons spawned alongside their horses and not on them, which was reported as another bug, which has since been fixed. As of 1.0.6.0, all the skeleton riders will spawn riding skeleton horses, and all of them will sink into the water and drown. The part where they sink because they don't know how to float while riding an entity has been reported as MCPE-20578.
So this ticket is either "cannot reproduce", "fixed", "invalid" for including multiple issues, or a "duplicate" of MCPE-202578. Hmm...
I think I'll just go with cannot reproduce.