Encounters with Giant Fish in Classic D&D
by HåvardSo, the easiest way to introduce an encounter with Giant Fish or similar monsters is if the characters are on a boat at sea, a lake or travelling on a river. However, even in those cases the players will often prefer to stay on the boat and unless the Fish monster is large enough to be a threat to the ship, there is little reason to get involved in a combat with them. For characters travelling across land, they might become aware of such creatures in a nearby lake or river, but again there might not be much reason for the PCs to care enough to get into a fight with them, unless they are murder hobos looking to kill anything that moves for XPs.
So what can be some situations where the PCs need to get into a combat with a Giant Fish?
1) The PCs are on a quest taking them to a Castle, dungeon or other location, but this location is found on an island in the middle of a lake. A flimsy boat lies by the shore, but there are nasty things in the water! Replace lake with river or swamp if needed!
2) Exploring a Castle near a body of water, the PCs discover that parts of the dungeon are submerged. They need to remove their armors and go for a swim to reach vital parts of the dungeon, perhaps to find a magical item or an important location. Surprise, surprise. Giant fish have also found their way into the submerged area.
3) The PCs have found the location of a sunken ship (or sunken Castle?) and must dive down to find a lost treasure.
4) Travelling through the wilderness, the PCs encounter an old man by the shore of a lake. He challenges them to capture the giant fish found in the lake. The old man might secretly be a Fey, Wizard or even an Immortal. Or just someone who enjoys fishing!
PC3 style Underwater campaigns obviously have no problems making use of this type of creatures.
What are other ideas?
5) The PCs hear a child calling for help. He/she comes stumbling up the hill from a nearby lake and says his brothers/sisters are being attacked by a monster. They were all in a boat, when something grabbed the oar and ripped it away from them. They tried to fight it off with the other oar and it stole that too. The child is soaking wet and shaking from swimming to the shore at top speed while being chased. He/she does not know any more.
6) The PCs need to cross a river at a ford. The water is unusually high and fast, as there are storms up river.
7) The PCs have to cross a neglected rope bridge. Giant fish leap out of the water to try to bite at them. Fish that miss the PCs have a chance to bite the rope the PCs are walking on, instead. If the fish bite through the rope, the bridge starts to unravel, leaving the PCs hanging onto ropes and dangling over the water.
8) The PCs board a rope ferry to cross a wide river. The ferry normally is pulled downstream, with the rope pulled tight, but something hits the ferry and pushes it against the flow of the river until the rope starts dangling in the water on both sides. Then the ropes on both sides suddenly get tugged from below the water. The PCs can try to help the ferryman to pull the ferry faster, but as the rope is pulled past the ferry, they see bite marks and eventually the rope is bitten through and they all float off down river. The ferryman tells the PCs there are rapids ten minutes down river and that they need to get the ferry to shore before then, or it will be destroyed.
A transport vessel appears in a body of slightly bigger than itself. A local king has heard of the vessel and knows its' contents; one of the cargo holds has been converted to an aquarium of large fish that a king wants to make a part of their exotic aquatic exhibit. The PCs are tasked with bringing the megafauna back to the king.
by barna10
I once used a rocky bridge over an underground river as this sort of scene. Anyone crossing the river was subject to jumping attacks by giant pike that would either bite them or hit them broadside and knock them into the river.
by BotWizo
While not in classic but in 1e we had a memorable fight with a Giant Gar that had my dwarf swallowed for a bit.
he picked up the nickname "the survivor" after that encounter which was kind of neat.