Giant Shark

A friendly giant shark is a stretch, but the framing writes itself once you commit. A sea hag's familiar that owes the party a favor. A sahuagin shaman has lent her temple guardian to escort the party past a kraken's reef. A druid in the party has bonded the shark through Awaken or a long-running compact. Whatever the setup, the shark is on loan, and it does not understand land. Anything that happens above the surface is the party's problem.

In combat, point the shark at one target and let it work. Blindsight 60 ft. means no concealment matters underwater, and swim 60 ft. closes most distances in a round. The shark grapples and pulls under, ignoring anyone the party isn't actively pointing it at. Don't expect coordination. With Intelligence 1 it doesn't form plans, it just pursues whoever the handler last gestured at.

Out of combat, the shark patrols. It can be told to circle the ship, watch the channel, escort a small craft. Anyone hostile in the water at night is a problem that solves itself by morning. The party should never see the kill, only the bloodstain on the hull at dawn. Play up the unease of having a 92-HP predator on retainer that doesn't ask questions.

When the loan ends, the shark leaves without ceremony. It is a beast with no language and no bond beyond the binding that put it here. If a player tries to befriend it personally, give them one good moment, a hand laid against the flank as it passes, and then it is gone into deeper water.

A giant shark is the reason the party shouldn't have left the rowboat. Run it as a hazard first and a stat block second. The shark's swim speed is 60 ft. and its walking speed is 5 ft., which means the encounter only exists in the water. Stage the fight where the party can't simply step onto a rock and stab down: a sinking ship, an open swim across a reef, a ritual platform that's already half-flooded.

The SRD entry here is sparse on actions, so the work is mostly positional. Huge size, AC 13, 92 HP, Strength 23, and Blindsight 60 ft. The shark closes the distance, grapples a target with an opposed Athletics check (it has +6 plus proficiency in practice through its size advantage), and submerges. From there, drowning rules and the swim-speed gap do the killing. Unarmored swimmers can keep up at 15 ft. per round if they make their Strength (Athletics) checks. The plate-armored fighter cannot. The shark only needs to drag a body two rounds deep to make the encounter terminal.

Target priority is whoever swims slowest. Blindsight 60 ft. ignores cover, darkness, and invisibility alike. A wizard who casts Invisibility and then tries to swim away gets hit anyway. Let the players figure that out by watching the shark turn toward an empty patch of water and strike. The Intelligence 1 means the shark does not feint or coordinate; it picks the closest bleeding thing and stays on it.

Sharks don't retreat from prey, but they do lose interest. Once it has dragged a body deep enough, it eats. If the party can give it something to chew on, a corpse, a sack of meat from the galley, a sacrificed mount, the rest of them get one turn to reach a hull or a beach. Make that trade visible and let them take it.