When sea mobs swim by any type of ‘seaweed’ they stop, and starts rolling around the weed they got below, after a couple seconds (finally) getting out to swim properly. It’s like they do detect seagrass as a normal block and try to void them some way, but should not be like that because it supposed to be a transparent block without collisions to detect by mobs. Most of the times, i see drowneds doing this movement at the second height block, over normal seaweed, or higher. I've seen this also happend with dolphins. 

Evidence:

https://youtu.be/tKLjC1mLJ9I