I'm planning an underwater adventure for D&D 5e that will contain an underwater portal to Dagon's layer of the Abyss (the 89th layer, called the Shadowsea, apparently). The portal will have demons spilling out of it, but it will be quite small at this point so it should be something the party should be able to stop; hence I want to know what demons would be thematic for being underwater and being associated with Dagon.
D&D 5e doesn't contain much information about Dagon (most of what I know of demon lords has come from running Out of the Abyss, in which Dagon does not feature, or the Monster Manual and Mordenkainen's Tome of Foes, neither of which mention Dagon at all as far I as can tell). The only things I know about Dagon come from the Forgotten Realms Wiki page on Dagon, which is not 5e specific and therefore draws on sources I don't have (I only have 5e materials).
This wiki page mentions:
Dagon was served by aquatic demons, especially wastrilith, [...]
But it doesn't further elaborate on what these other aquatic demons are.
The only appropriate demon I've come across so far is the wastrilith, mentioned above, which appears in 5e in Mordenkainen's Tome of Foes (since it is associated with water, though the book mentions no relation to Dagon specifically, but that Forgotten Realms Wiki link does), but it's CR is quite high for the party level range I'm planning. There is also the hydroloth from the same book, but that's a yugoloth, not a demon, and I'd prefer demons where possible.
What demons would thematically follow Dagon?
Ideally of a lower CR than a wastrilith and ideally from 5e where possible, but neither of those are hard constraints. I'll accept anything from any edition of D&D.
Related, but about Baphomet: What demons are under the dominion of Baphomet?
Related, but about Juiblex and Zuggtmoy: What demons are related to the demon lords Juiblex and Zuggtmoy?
Unlike that first question, I'm only interested in demons specifically (unless the creature has business existing in the Abyss, such as a yugoloth; I assume that pretty much only demons would be there).