These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Goddamned Bats: Most enemy fishes can qualify as this, especially those who will not stop chasing you until they finally eat you. However, there are a few... glaring examples.
Pufferfish can, well, puff up and bounce you off. Not only do they tend to do this at the worst times, and even when you sneak up on them like the game suggests you do, but they reduce points, and give no Mercy Invincibility. If you're bouncing straight into a enemy fish... There's no help for you.
Barracudas in the original were rather annoying, as they cannot be eaten no matter what you do. (Well, unless you count the shark king, who is a rehash of them in the final level, and even then they didn't do anything other than dissapear.) It wasn't until the 2nd game they were finally able to be eaten, and they gave rewards as well.
Sharks/Orcas are uneatable NO MATTER WHAT. You must let them pass along, or hope they get caught in a clam or get hit by a cuttle fish's stray ink. In Feeding Frenzy 2, however, you can use a shrink mushroom... and you get to be a shark at one point, who can gobble them up at his highest size.
Cuttlefish can make your movement rather erratic and odd to control. There's a reason many players have an Oh, Crap at levels where you MUST eat them to advance. (And yes, they do exist.)
Poison minnows reverse your controls, and that's it. These can actually be removed faster by rapid clicking left, but you better hope you don't overestimate how long it takes, and if you're planted in front of a mine, you'll dash into it and kapoom.