Barraskewda-
With 123/136 Attack and Speed, the Swift Swim Ability, Barraskewda make a perfect Physical Sweeper, especially with Life Orb. With access to strong moves like Liquidation, Crunch, Close Combat, Poison Jab, Drill Run and many more. These help Barraskewda become a Potent sweeper. However, Barraskewda's bulk is minimal, but it is still a great option.
Seismitoad-
Seismitoad also benefits from Swift Swim, but has lower Attack and Speed. However, where Barraskewda lacks in bulk, Seismitoad makes up for it. Having one weakness, 105/75/75 bulk, Seismitoad can help sweep.
Poliwrath-
Poliwrath is a lot like Seismitoad. Swift Swim essentially gives it a 140 base Speed Stat. Poliwrath also gets options to set up with Bulk Up. It also gets reliable STAB moves in Close Combat, Drain Punch and Liquidation.
These are major Water Types that benefit from Rain that are viable in OU with Rain set up. Now, here are some non-water types.
Scizor -
With Rain set up, this makes Scizor's Fire weakness much more manageable. With Technician, Scizor gets great STAB moves in Bullet Punch, U-Turn and Knock Off. Now, Scizor also gets recovery in Roost, and Swords Dance for sweeping abilities.
Toxicroak-
With Dry Skin and Black Sludge, Toxicroak can easily set up with Bulk Up or Swords Dance. Also, with access to further healing in Drain Punch, along with STAB in Gunk Shot, and Priority in Sucker Punch, Toxicroak can be a viable Sweeper. Toxicroak also gets a nice move in Fake Out.
Since you also clarified, I will include some Pokémon that help.
Excadrill-
Excadrill gets Mold Breaker Earthquake to deal with common Electric types like Rotom-H and Zeraora. Excadrill also benefits with its lessened Fire Weakness. Excedrill also gets Poison Jab to deal with Grass Types.
Ferrothorn-
While Ferrothorn isn't necessarily Physical, it still use Gyro Ball or Power Whip as its main attacking moves. Ferrothorn easily deals with common coverage moves like Thunderbolt and Energy Ball that wreck some Rain teams.
Others- Pinchurin with Lightning Rod. Azumarill with Sap Sipper among others. Please comment if I missed anything.
Other Pokémon that benefit: Aegislash, Zarude can also benefit, Gravity-Rapid Strike with boosted Surging Strikes. Hopefully this helps! Feel free to comment if you feel I missed an important one because I probably did. Also, I believe Barraskewda is by far the best option for Water Types.
