Here's a recreation of Boss Bass from Super Mario Bros. 3. It pretty much acts how you'd expect it to, swimming back and forth below the player and jumping out in an attempt to eat them. It's not an exact replica of it's original version, but it's decently close.
This is awesome! It's very accurate to the original and acts exactly the same, only less accurate thing is it falls a bit slower than the original SMB3 boss bass, though it's not an issue. Good job!
I noticed two issues in the code of the enemy. It basically dies if it tries to jump when within block collision; Semi-solid and solid alike.
You sure something isn't messed up with other code in your level? It shouldn't be freezing randomly or failing to not float in the water, and it's doing neither of those things on my end.
Edit: Updated link to make the NPC immune to iceballs, since being frozen then unfrozen seems to cause collision issues.
I see you figured out the problem that other guy was having with his Boss Bass npc since I don't think he could figure out how to give it a custom death animation for when it eats the player which is why it only hurt the player like any enemy would.