Giant Octopus

A giant octopus ally is the answer to one specific GM question: how does the party get into the sunken thing? A sea hag has loaned them one. A druid has befriended a colony of octopuses on the reef. A merrow merchant rents out trained ones by the hour. The octopus does not bond with the party. It does the job and goes back to its hole.

Use it as an underwater mount and recovery tool. Swim 60 ft. is faster than nearly any PC's swim speed. The octopus carries a Medium rider through deep water without the rider needing to roll Athletics, which trivializes a long approach to an underwater dungeon. It can also retrieve dropped items from depths the party can't reach without magic, scout ahead and signal back with a colour change (rule it as a one-bit semaphore: red for danger, blue for safe), and squeeze through gaps the party cannot.

In combat, the octopus is fragile (45 HP, AC 11) and limited to a tentacle grapple. Use it to lock down one underwater enemy at a time, freeing the party's melee to focus on others. Don't expect it to last more than two rounds against anything that can hit back. Have it disengage to deeper water once Bloodied and let the party finish the fight.

When the loan ends, the octopus drifts away on the current. If the party wants it back later, they have to find the same patch of reef and offer something better than the last bribe.

A giant octopus is a coastal ambush at CR 1 with no listed actions, which means you're running it on Stealth +5, Strength 17, and a ruling. Treat its attack as a tentacle slap or a grapple. The interesting fight is positional: the octopus wants to drag a PC underwater and hold them there until they drown, not trade hits on a deck.

Set the scene with the octopus already submerged. Passive Perception 14 plus Stealth +5 in murky water means the party rolls Perception against a fixed DC of around 15 to spot it. Walk 10 ft. and Swim 60 ft. tells you the octopus stays in water and only crawls onto a boat for the half-second it takes to grab someone. Use a tentacle as a Grapple via Athletics +5 (escape DC 13), no damage on the contest, then Swim 60 ft. into deep water dragging the PC. The drowning rules do the work the stat block doesn't.

The octopus is not a brawler. AC 11, 45 HP, no multiattack. If the party gets a clean line with ranged attacks, it dies in two rounds. Its survival depends on cover and on a quick retreat into ink (rule a Hide action behind an ink cloud as a once-per-encounter tool if you want the flavor). Once Bloodied, it disengages to deep water and leaves the party with whatever PC it grabbed, alive if they made their saves and dead if they didn't.

Octopuses flee when the math turns. INT 5 is low for a humanoid but exceptional for a beast. The octopus will not engage a party that has its boat surrounded by torches and ready spears.

Have the octopus take one specific item before it leaves: the bard's hat, the ranger's spell focus, a sealed letter. The party gets to decide whether to follow it back into the water for the recovery dive.