The version of Sea Elf in Mordenkainen says, "you can breathe air and water", but doesnt explain how this happens. Presumably it is a magical feature, but not explicitly a spell feature.
Coming afterward, 2024 has a new more systematic format for the Elf species as a whole.
It is possible to update the Sea Elf along with other elven cultures according to the new Elf format.
In this case, the elven waterbreathing feature might be by means of a spell.
Keep in mind, 5e has three different versions of the Eladrin culture. One in the 2014 DMs Guide. One in Mordenkainens Tome of Foes. One in Mordenkainen Presents Monsters of the Multiverse. There can easily be a fourth version as an update to correspond to the 2024 core rules. A DM can choose to use any of these versions for a player Eladrin Elf.