Now this is an interesting one. Well, several really. I've talked before about aquatic encounters of various types, but it seems that whole topic is incomplete without discussing this listing.
Giant fish covers unusually large versions of more or less real-world fish; including piranha, rockfish, catfish, and sturgeon. When the books says "giant", it means it! These are some big guppies! Unsurprisingly, none of these are found wandering in dungeons, they are either wilderness or placed encounters only.
Let's put the stat table up for a peek, shall we?
Giant Fish (from Cook)
Just looking at things like AC, HD, and damage, these are
not trivial encounters. Especially when the added challenge of
most such confrontations would be in or under the water.
Giant Piranha: Let's just take a moment to appreciate the fact that the "wimpy" one on this list is a GIANT PIRANHA which, by the by, are five feet long! Growing up in the 70s, I was personally convinced the three most likely ways nature would take me out would be: Shark, Piranha, or Killer Bees. And those piranha were less than a foot long! Moving on, these monsters aren't unstoppable brutes, but even 3+ HD means they aren't something you just one-hit away. Couple that with their morale-less frenzy and up to eight of them on a single target dealing 1d8 each, that's a bad time for their fella that gets ganged up on.
Giant Spiny Rockfish: While the listing doesn't specify how big this fish is, given that it has more HD than the piranha and is sometimes mistaken for boulders, at least 6' seems not unreasonable. It's size isn't really the issue. The fact that one might not recognize it and get to close is the problem. Not only does it have a poison attack, it had FOUR of these per round! The spines do 1d4 regardless, so up to 16 damage is still a cause for concern. The spines are melee only, but if you don't realize what your dealing with there in the shallows and actually step on or grab hold of that "rock", the fish automatically hits all four times! And yes, that means FOUR save or die rolls. It's also worth noting that the poison gives no "time until effect" meaning an insta-kill. It's AC isn't great, though, so it's not exactly unkillable. It also has a skittish morale, as befits a creature that relies on camouflage. What's odd about the listing in Cook is that it has no asterisk next to the HD denoting bonus XP for extra abilities. 4 attacks with poison would seem to merit that in my opinion.
Giant Catfish: Now things are getting serious. Fifteen feet long! Eight-plus hit dice! 2d8 bite damage! The giant cat's big guns are its feelers, though. Those bring it up to FIVE attacks and up to 32hp damage dealt in one round! Again, not a great morale score, but a markedly better AC than the earlier entries.
Giant Sturgeon: Oof! This thing approaches dragon levels of difficulty. 30' long, 10+ HD, and AC 0! What's more, they swallow whole 10% of the time and the victim stands a fair chance of being immobilized while taking 2d6 damage per round in the fish's gullet. And that's after taking 2d10 damage from the bite!
What's the worst thing about all of these encounters, you ask? It's not the damage per round, or the fact that you're probably fighting them in their element (and out of yours), it's the treasure! Rather, the lack thereof. Imagine having to battle your way clear from a school of piranha, or kill a rockfish after it poisoned a party member, or gut a PC's corpse out of a sturgeon gut, only to get not a single copper piece from the monster toward the raise dead fund! talk about a poor ROI.
Looking at these creatures less as monstrous foes and more like dangerous, fantastic animals that are part of your game-world probably casts them in a more accurate light for the PCs. It's like encountering a bear in the woods. Chances are, he's not why you're there. Be careful, keep your distance, and things will probably be fine. Act recklessly or be inattentive, and there could be consequences.