@Scribe,
@Remathilis
There is a kind of compromise, sotospeak. The "mythal".
The mythal is a powerful collective magic ritual that an entire elven community participates in. A mythal can be for any magical purpose.
Plausibly, the elven community performed a mythal to magically grant waterbreathing and swim speed to every member of the community. The Sea Elves ancestors performed this mythal to inhabit the underwater.
Therefore.
A cultural background feat (at level 0) can grant mythal features, such as the
Gills cantrip and swim speed.
Something like.
SEA ELF MYTHAL
Background Feat
Prerequisite: None
You were born in a Sea Elf community, or became a citizen of one. The collective magic of a mythal there transforms you for life underwater.
Child of the Sea. You gain the
Gills cantrip and can cast it innately without a slot. You can do so again after a Long Rest. You are immune to harsh frigid environments, and exhibit Resistance against magical Cold damage.
GILLS
Transmutation Cantrip
Casting Time: 1 action
Range: Self
Components: S
Duration: 24 hours
You transmogrify your body to adapt to life underwater, sprouting gills and undulating body. You can breathe air and water. You gain a Swim Speed equal to your Speed.