Aquatic elves are mentioned under the sahuagin entry in the Monster Manual, and also in Ghosts of Saltmarsh, but I don't see them defined anywhere. Is there a stat block or other description of them anywhere beyond the sahuagin entry?
Mordenkainen’s Tome of Foes pages 62-63.
A description and set of racial traits for Sea Elves is found on pages 62 and 63 of Mordenkainen’s Tome of Foes, as well as a brief description (including racial traits) on pages 163-164 of Explorer’s Guide to Wildemount.
This information can also be found here at DNDBeyond, but requires that you purchase MToF or at least the Sea Elf subrace.
A basic monster stat block for a sea elf can be found on page 70 of the adventure Storm King’s Thunder, and Ghost's of Saltmarsh contains a statblock for a named sea elf, Oceanus.