Do we need to re-install tiles if it was exposed to water for 20 minutes after installation?
There are two problems I can think of. The first is exposing the thinset to excess moisture, this will impact the chemical reaction that's part of the curing process, which may lead to a weaker bond and loose tiles. It's easy enough to wait at least 24 hours and then check the tiles to see if they are firmly in place. If a bunch are easily dislodged, then I'd remove everything to inspect for damage behind the walls. You'll want a new backer board for a flat tile install anyway.

The bigger problem is if water got behind the tiles into the wall. A proper install will have a waterproof barrier behind the tiles since grout, thin set, and often the tiles themselves are not waterproof. If this barrier (e.g. Ditra) isn't the case in your install, the materials behind the tile and possible paths water could take are important. If water is getting into drywall, even green board, wood (studs, plywood, or OSB), or on metal (non-galvanized nails/screws, electrical lines, communications lines) you will see damage. The damage will likely take time to detect, e.g. rusting, corrosion, and mold won't be noticeable immediately.

On the flip side, it's also possible that any damage that has resulted will not be noticed until after you sell the home or perform another renovation. Because of this, it's difficult for anyone to give a definitive answer. You may be exposing yourself to significant health hazards from mold, or you may not have any issues at all.