Water mobs can't be placed in water buckets on realms. I have tested this with tadpoles and axolotls on multiple worlds in my realm, it works as intended in the regular game.

Steps to reproduce:
1. Get an axolotl bucket or tadpole bucket
2. Place down the mob, preferably in a hole so it can be contained
3. Collect the water back in your bucket
4. Attempt to pick up the mob with your water bucket

Expected Result:
Mob is picked up with water bucket

Observed Result:
Water bucket doesn't collect the mob no matter how many times you try