I'm trying to obtain all the possible moons before beating the game, and the last obtainable one is at the small painting island. It's too far away and I think the only way to get there would be by doing the swim on air glitch, but I can't do it from any of the 2 small pools. Has anyone managed to do ti?

I think that requires a frog

There are 2 problems.

1. We don't have a way to do this consistently without a frog. A rare incident.
2. Even if you were to airswim, I doubt it would let you reach the island anyway, whether this be running out of air, or the body of 'water' not being large enough to get you there.

I think you can use the peach amiibo to refill your air meter

Apparently, if you swim fast enough, you reach air before going back in water. Other problems arise though.

We don't know if it can happen without a large quantity of water. (it might happen, but the airswim glitch isn't common enough to really be able to tell, as people will miss the tiny lakes around) The water in the air has a height limit, and a strict X and Z axis limit. Go over land, and there's no 'water' over it. Would it be enough to get to a height such that you can jump out, and reach any 'out of reach' spots? I doubt it a heap.

I do hope I end up being wrong though.