Thanks for playing!
So regarding the combat, it largely stems from the AI navigation trying to get onto the same tile as the player when it's supposed to be trying to reach the tile NEXT to the player. The reason you couldn't hit it is because the raycast for melee attacks and the projectiles doesn't count any colliders that the raycast starts in. I wasn't able to sort that problem out before the jam ended, and in general in the last two days I decided to focus on everything else but the combat because I figured it would be better to have a strong atmosphere with weak combat, than ok combat with a boring bland world.
So for the sound effect I guess in my mind it sounded like a good stab sound for an underwater attack, but most players either didn't notice it, or as you mentioned it sounds like a miss. There is a visual indicator for a hit with the little blod puffs that come off enemies, but they end up being obscured by the enemy models, and for the zombies and mermaids the blood colour is blue which I only realized afterwards blend into the background too much.
Regarding the ghost enemies I do know what can cause that. When the game starts, an A* map is built from every tile in the game, and then each tile raycasts in the 4 cardinal directions to see what tiles it can connect to. But this raycast, like the attack raycasts mentioned earlier, doesn't detect colliders the raycast starts in. So the tiles next to a giant world object don't detect a navigable tile, but the tile holding the impassable object DOES. And because AI navigation follows the map but not raycasts, they can just walk through it.
I'm glad to hear the shark was appropriately spooky though. You weren't imagining things with it coming back to life, I saw it happen on stream actually. I'm not sure why it happens, but there's only one shark enemy in the game.
Thanks again for the feedback! Sorry the combat was so confusing, I'll have to sort that out post-jam and keep it in mind for next year,