Passive land mobs (tested on chicken, pigs, sheep and cows) will get stuck when going through water and passing by a seagrass block. Instead of going forward they will spin on themselves and cannot move out of the block until pushed by the player.

Steps to reproduce : 

1. Set up a rectangular zone with 1 layer of source water blocks, and place seagrass on some of those blocks.
2. Spawn a passive mob and attract it towards you using the correct item (wheat/seed/carrot...).
3. Move around to make the mob pass through seagrass. It should get stuck.