Hi there Helena
I used to work for the Nintendo Service centre UK, and can tell you that water damage is not covered under warranty. there is a little white paper sticker in the battery enclosure (if you open the back case and take the battery out). if this white sticker, has red dots or lines on it, then its considered "water damage". that is what they look for.
The advice i usually give for any water damaged electronics, is remove the battery, as soon as it gets wet, and open it to let it dry.
if there is no current running through the circuits then water doesnt really affect electronics (other than physically making them wet. if you get it to dry properly, and then put the power back in, it may still work.
But im afraid in your case, you may have even charged it whilst it was still wet inside, and this causes 'short circuits' in the electronics (because water is a conductor of electricity), and this may well have 'killed' it.
My suggestion: try taking out the battery, and opening it, to get it to dry out fully, before attempting to charge it again. if its still dead, then (if you are feeling brave), look on ebay, for a DSiXL, with maybe a broken screen, and buy it for spares. chances are good that the water hasn't damaged the LCD screens, and you can use those to replace a DSi with physically broken screens.
there should be a few step by step guides on here on how to replace screens or just take it apart.
I hope this helps