Currently, tails angle up while sprint-swimming underwater, and it looks a bit silly and un-streamlined.
Legs don't swing nearly as far in both water-walking and sprint-swim animations, compared to walking on land. Should be plenty of room to avoid clipping.

Pics:

Sprint-swimming and moving:

Not moving, in the same pose:

Great idea!

This is because Ears Common doesn't know what swimming is and just detects it as sprinting. Ears has become too much of a mess at this point to make a change to every 1.13+ platform port, so this will have to wait for Ears 2.

Tails 2: Furtastic Boogaloo!