1. If it's something like an innate normal ability, then she won't need anything special! She can just be able to breathe underwater from the get go, won't even need a tattoo. If it is something on the more magical side, like she has to cast a spell for it, then she'd need the tattoo at least. As for how long, that might depend on how she is originally. If she could breathe underwater for an indefinite amount, then just to make things interesting, we'd say her limit would start at an hour or two, but this can gradually increase over time.

2. Yes, that's fine!