Kind of duplicated by
MC-93855 - which is now marked as "resolved" despite the bug still existing in the latest versions/snapshots?
Skeletons' bodies are limited to a 180 arc when in boats, but can still fire backwards at mobs/players (which also causes a weird arm/bow-loading animation)
Skeletons will also attempt to shoot at players boarded in the same boat as them, however the arrows only appear briefly behind their heads [?] without damaging the player in the boat or sticking into any nearby blocks.
How to reproduce:
1) Spawn a skeleton in a boat
2A) spawn a baby turtle somewhere behind the skeleton OR enter survival mode and stand behind it
2B) Board the boat with the skeleton and enter survival mode