Sea Spawn is a name that can refer to two (technically three) very different aquatic monsters in the Dungeons & Dragons multiverse. The older version is native to the Demiplane of Dread, whilst the younger is a more recent discovery and inhabits various worlds of the Prime Material. The two should not be confused, because they have very different natures and powers.
Ravenloftian Sea Spawn
The Sea Spawn native to the Demiplane of Dread first appeared in Advanced Dungeons & Dragons 2nd edition; it was one of the many monsters gathered from Ravenloft adventure modules and compiled into the Ravenloft Monstrous Compendium Appendix III. It was subsequently converted into Dungeons & Dragons 3rd Edition, appearing in Denizens of Darkness (3.0) and Denizens of Dread (3.5).
The Ravenloftian Sea Spawn is a vile predatory aquatic monstrosity - even being typed as an aberration in 3e - that is described as resembling a piscine humanoid with oversized, webbed, hands and feet tipped with claws, as well as a thick coating of filthsome, noxious slime. Yes, this is yet another fish-man. Weirdly, their artwork in Denizens of Dread depicts them with a merfolk like body-arrangement. Anyway, this actually only describes the adult form, or the "Sea Spawn Master". In their larval state, known as "Sea Spawn Minions", these Sea Spawn resemble oversized (six-inches long) slugs or eels, with mouths like those of a lamprey. The adult form is strictly aquatic, and will quickly suffocate outside of the seawater in which it lives, but the larvae are more resistant, thanks to their thick coats of slime. Ravenloftian Sea Spawn reproduce asexually, with "masters" producing broods of 3d6 "minions" per night when they deign to spawn. The vast majority of minions dies; only 1% will survive the year-long period it takes to mature into a fully-formed adult.
Sea Spawn Masters earn their nickname because they are naturally telepathic, able to communicate with and control their spawn and other natural aquatic creatures. They are feared in all the coastal regions where they occur, because they are natural predators of coastal towns. Their Minion larvae are sent to slither up onto docks and piers before making their way into the community at night; they use their paralytic venom to immobilize sleeping humanoids, and then burrow into the base of the skull so they can wrap around the brain-stem. This allows the Minion to control its host and access its memories; hosts become more reclusive and emotionally detached, but this is the only warning sign. As it waits, the Minion slowly consumes brain tissue, causing the victim to lose 1 point of Intelligence per day; in AD&D, this can't be cured by anything other than a Wish spell, whilst in 3e, it's classified as Ability Drain. Hosts reduced to 0 Intelligence promptly die. Minions can only be excised by the use of Cure Disease or Restoration spells, but this causes them to violently chew their way out of the host's skull in their urge to escape, causing 3d6 damage and probably killing them - 3e allows the Greater Restoration spell to kill a a Sea Spawn Minion without hurting its host.
So, what terrible plots do Sea Spawn get up to, using this slow and insidious domination of a town's population? ...Hunting. Yeah, despite what you'd think, whilst the Sea Spawn Master is smarter than a mere animal (it ranks an Average on the Intelligence rating in AD&D, and is Int 12 in 3e), its entire motivation lies solely in the fact that it thinks humanoid flesh tastes better than fish. As its Minions take over more and more humanoid hosts, they use their host bodies to abduct the weak, vulnerable or easily disposed of - children, vagrants, things like that - and throw them into the sea for the Master to eat. If they can't find them, or the Master gets impatient, a random host will jump in and be devoured - larvae and all. Ultimately, if not destroyed, the Sea Spawn Master will enthrall and devour the entire population, before moving on to find a new town to harvest.
...Yeah. They're not exactly the most exciting of monsters. Great for a one-off encounter, but they get old quick. Much like a lot of Ravenloft monsters, really.
Great Wheel Sea Spawn
The Dungeons & Dragons 5th Edition version of the Sea Spawn debuted in "Volo's Guide to Monsters". Replacements for the long-neglected Skum, they are similar to the Deep Scions also introduced in that book, in that they are humans warped into malformed amphibious versions of themselves by cruel undersea spellcasters. Whereas Deep Scions are shapeshifting agents above the water, Sea Spawn are totally aquatic and serve as drudge laborers beneath the waves.