The core book has the MuAq spell Lungs of the Fish, which "turns water to air as it enters your lungs, allowing you to breathe water as you do air." Level 20 for R: Touch, D: Sun, T: Part, but if you make it R: Personal, that drops it to 15.
Personally, I prefer a nice Muto Corpus effect. Base 2 to "change someone to give them a minor ability." There's not a corresponding level to give them a "major" ability, but Level 3 utterly changes their appearance, so I'm thinking either 2 or 3 (I'm leaning toward 2). R: Personal, D: Sun (+2), T: Individual, would make it Level 4 or 5. For comparison, Eyes of the Cat is level 5.
Or I could zip over and look up the Essence of Gillweed that Fiona has. Yep. Base 2. Which, if anyone has been chatting with Moire (who talks to anyone and everyone, since she's a gossip) or Seumas, has a pretty good chance of this having been mentioned. Especially since Seumas mentioned it when Jormungand was investigating the flood last fall.