Well for starters, a stainless steel revolver is one which will resist rust the most and be easiest to clean up after being submerged. However, all firearms can handle water submersion as long as proper cleaning, drying and lubrication is performed in a timely fashion after getting it wet. You can also keep it in a water tight container such as an Aloksak bag or similar while on the water.
I've carried an XD9sc, Ruger LCP, and Stainless Ruger SP-101 revolver at various times on various float trips in a pocket holster which was then stored inside a waterproof Aloksak ziploc bag, and then kept in the cargo pocket of my shorts which was buttoned down. And getting totally submerged while swimming on float trips, not a sign of moisture has ever gotten in them.
Still if they were totally unprotected and got dunked they'd still be fine if you stripped them down and dried them out using towels and Q-Tips and then lubricated. Also canned air or an air compressor if available works well with dissipating moisture from the internals.
As far as ammo goes... be sure to dry each round individually with a dry towel after getting wet.