I think that physically offensive Water types have a lot to gain from this metagame. Unless you're Crawdaunt, typically the best physical Water attack you can manage is Waterfall -- 80 base is kind of pathetic especially compared to special Water moves (Scald is the same power but has a 30% burn chance) or physical offerings for other types (Close Combat, Brave Bird, Earthquake). Here are a few notable improvements now that physical Water STAB can be 100-120 bp:
Gyarados has been mentioned, but the DD Mega set is especially good now. Not only can you use a move like Return/Earthquake as your water STAB, but you can play the usual trick of remaining in base form until needed, only now you have access to reliable flying STAB. Once you Mega-evolve, that same move is now Dark STAB. This essentially gives Gyarados the coverage of four attacks while still running Dragon Dance.
Kabutops is a menace in the rain, and now has a lot more versatility. Along with the standard Return/Earthquake, Lord Dome has access to Knock Off, Low Kick and Superpower, you can both replace your weak Water STAB and inaccurate Rock STAB with moves that have added benefits like removing items, punishing heavy targets and being 120 base power. You can also run Rock type Aqua Jet to pick off weakened foes or unspinblockable Rapid Spin if you're weird.
Azumaril is a similar story -- not only can it use more powerful AND accurate Water/Fairy STAB, but it can use its secondary Fairy type for Aqua Jet priority as well as utilizing Superpower, Double Edge and Knock Off as STAB. Banded, AV and even Belly Drum sets were already considered top tier, so Azumaril might be one of the biggest threats around.
Mega Swampert is self-explanatory: rain-boosted 102-120 base power Water STAB hurts coming off 150 base attack. I've seen a few Power-Up Punch sets floating around in OU, so you could be cheeky and stick it on your second slot to get STAB while sticking Earthquake in the third or fourth slot.
Mega Sharpedo's Waterfall has always felt a little weak in comparison to Crunch, but now you have two options: keep Crunch as Dark type and replace Waterfall with a higher base power move, OR give yourself a Water type Strong Jaw boosted Crunch in your first slot, along with 100 bp+ Dark type move in slot two. It depends on what type you need to hit harder.
Bibarel with Curse and Simple though
Greninja has always had a few physical sets floating around, but they've been criticized for low bp moves in comparison to special sets. No longer! With only a marginally weaker physical attack stat, Water or Dark type Return is nearly comparable to Hydro Pump and Dark Pulse. U-turn has remained a niche move on Greninja but with more incentive to run high Attack EVs and access to superior Water and Dark typings, it does a lot more than chip damage. You still have the last two slots to take advantage of Protean, so enjoy your Jolly Gunk Shot and Low Kick!
Barbaracle is...still probably trash, but one its main problems is its inaccurate, low power moves. Seriously, you thought Waterfall was bad? All this guy has is Razor Shell. Not only that, but Stone Edge doesn't even benefit from his ability. +2 Tough Claws 102-120 bp (if using Superpower) Rock or Water STAB begins to hurt a lot more than you'd expect, although I feel its still outclassed by Kabutops. Still, Barbaracle isn't reliant on rain in order to get a speed boost to pull off a sweep, so it might be worth giving it a shot.
Finally, while it is neither a physical attacker nor a Water type, I'd like to give special mention to:
Mega Sceptile suffers from an epidemic we all know and struggle with: 4mss. It wants Leaf Storm to do relevant damage, but also wants Giga Drain for recovery. As Dragon Pulse is mandatory, you're limited to one coverage move if you choose for dual Grass STAB, so most sets choose one or the other. Now you can either choose Dragon type Giga Drain (which has much better neutral coverage and thus restores more health on average) along with Grass type Leaf Storm OR
Draco Meteor Dragon type Leaf Strom for a powerful nuke alongside standard Giga Drain. This leaves two spots open for Hidden Power/Focus Blast/EQ/Sub.