The ocean food web
The sea contains salmon, trout, reef fish, jellyfish, bottom-walking crabs, sharks, blue whales, octopus, squid, rays, orcas, belugas, krill, anglerfish, nautiluses and the rare kraken. They do not all use identical depth bands.
Sharks seek fish, octopus/squid, rays and mermaids. Blue whales eat krill. Octopus/squid eat fish and krill; rays eat krill. Orcas hunt fish/octopus/squid. Belugas eat fish and krill. The current orca food web does not target mermaids or people. Do not infer aggression just from a real-world species' reputation.
Sharks and rays
Sharks can notice Mortal swimmers within about 17 blocks and bite within about 2.3 blocks for eight health damage. Natural spawning checks deeper water and margins from shallow coast. Explicit spawn eggs do not inherit that habitat safety promise. A ray can deal three damage through very close contact, within about one block in Mortal.
One creature with two forms
Mermaid and Siren are day/night forms of one persistent creature. Old Siren IDs/eggs canonicalize to Mermaid. Do not count them as two independent natural species.
By day a calm mermaid can help a nearby drowning Mortal player/companion rise toward the surface. It carries no trident in the current game. It can fight hostile mobs in water, and a provoked daytime mermaid can strike once before warning and retreating. Assistance is conditional, not guaranteed oxygen.
At night it approaches underwater in daytime appearance. It begins a lure within about six blocks; merely being nearby does not reveal the siren. After the lure, approaching within about one and a half blocks enables grasp, reveal and biting. The camera stays usable, swimming is restrained and oxygen continues dropping.
Escape a siren
Click separate melee strikes. Three normal bare-hand hits break the grip. Existing melee strength matters: a stone pickaxe needs two and a strong blade can need one. Holding the button does not automatically struggle. An ally can help by melee or arrows. Escape gives a six-second retreat interval.
Daylight, Creator mode, death, losing the underwater target or active Twilight-crystal protection cancels a hold. Grips are transient and do not resume from saves. Two sirens cannot hold one swimmer simultaneously. Nearby sharks interrupt marine interactions and drive the creature to flee; bringing another predator is not a safe escape method.
A beluga ride
Aim at a beluga within about four blocks and right-click raw/cooked salmon, raw/cooked trout or krill. Then select an empty hand and right-click it. Hold W while looking where you want to go; Shift lets go. This is ride permission rather than standard wolf/cat taming.
A beluga can also approach a low-oxygen Mortal swimmer/companion and give an upward boost. Oxygen still matters while riding: you recover if your eye is above water and lose oxygen if submerged.
Ink and deep wildlife
Hurt octopus/squid emit ink and flee. A valid unconsumed kill can provide raw squid for kiln cooking. Krill are wildlife as well as an obtainable item.
Anglerfish and nautiluses naturally appear near the floor in very deep loaded water and move slowly. Kraken are exceptionally rare in deep offshore habitat with a surrounding depth margin. A kraken starts at 480 health, can hunt pirates/nearby swimmers, and avoids pursuing into shallow water. Close body contact remains dangerous; manually placing one in shallow water does not make it safe.
Ocean eggs must be used in water. Creator is a convenient observation mode that removes the player from targeting while retaining creature-to-creature interactions.