[@Aerandir] Hello there! :)

Here are the weaknesses to both;

1. I was thinking that their body would be more accustom is living in the water, so they'd constantly have to keep themselves hydrated and the like. Using the second part of their quirk would result in losing water a lot fast than they would normally. The body parts they change into water are still their bones and flesh so losing water (via fire or extreme heat or something) would be the same as losing said body part until enough water is ingested to reform the limb.

2. Kinda mentioned a little about before. So no overlapping of tattoos. The weight of each object they put inside of them is the same, so the more stuff they have in them, the heavier they get. The properties of said items are still there, so things like metal detectors will go off on her tattoos. And if a tattoo were to get cut or anything of the sort, said item will morph into their skin, causing excruciating pain and maybe even deadly consequences depending on the item. (The more I type about this, the more I wanna go with it lol)

It would be legs with fins instead of toes, if that makes sense? So they can walk and still be able to swim ^^