Guardians and elder guardians captured in minecarts by players that are in survival would try and swim while them and the minecarts are submerged in an ocean.

To reproduce (In survival)

1. Put a guardian or elder guardian in a minecart
2. Let the minecart with the mob sink into the ocean
   → The guardian/elder guardian retract their spines and try to swim while in a minecart.

To reproduce (In creative)

1. Run the commands while underwater:

    /summon minecraft:minecart ~ ~ ~ {Passengers:[{id:elder_guardian}]} 

    /summon minecraft:minecart ~ ~ ~ {Passengers:[{id:guardian}]}