We left it vague on purpose; only Crabs/Witches/Wizards are explicitly sentient.
If your group is excited to talk to fish, then fish should be sentient (if a little dumb)! Ask your players what they would enjoy most.
Pg. 17 has some more examples. "Skeletons can also communicate ideas/emotions/images via touch. This allows them to “talk to fish” in a way."