Phone immersed in water and now won't turn on or charge

Your phone will have to be replaced most likely.

You can leave it in a cool dry place for a week or so and see if it powers on.

If it does, make frequent backups as it most like will fail soon due to corrosion.

What johnathan offered is good advice. Water damage can't be fixed. You will most likely need to buy an Out of Warranty replacement or a new phone if you're ready to move up.