Hi, how much for water damaged (under water) repair cost for Ipad pro 11" (Gen 3) please?

Water and electronics do not make happy bedfellows. Your iPad is not designed to be water resistant. While it may survive being splashed with water if immediately dried, it will almost certainly suffer irreparable damage if submerged.

You would be best advised to take your iPad along to the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to assess your iPad. If repair isn’t possible, the iPad can be responsibly recycled - and you’ll be in the best possible place to explore possible replacement:

Genius Bar Reservation and Apple Support Options - Apple

See this for a guide, iPad Repair & Service - Apple Support

truthfully, you’re better off buying a new device as water gets everywhere