Hydro Cannon: Make the water starters great again!
Blastoise has always been a worse Vaporeon, with 171 attack, vs Vaporeon's 205. Feraligatr on the other hand, has 205, the same as Vaporeon, but less bulk. All of them have the same best water-type moveset. Gen 3's Swampert has 208 attack (slightly more than Vaporeon), but similar bulk to Feraligatr, and additional ground typing.
So, this is my suggestion to add Hydro Cannon, the signature move of water-type starters, into PoGo. It should be a very powerful single bar water-type move. Even if it's a totally broken move, it's fine because the 3 water starters don't have very high attack stat to start with.
Let's say it's a water-type Overheat.
Blastoise:
Fast: Bubble, Water Gun
Charge: Hydro Cannon, Ice Beam, Bulldoze
Feraligatr:
Fast: Water Gun, Waterfall
Charge: Hydro Cannon, Aqua Tail, Surf
Swampert
Fast: Mud Shot, Waterfall
Charge: Bulldoze, Earthquake, Hydro Cannon
Blastoise's attack stat isn't enough to do much. However with Bubble and type-coverage moves Ice Beam and Bulldoze, it can be a decent defender in high turnover areas.
Feraligatr currently doesn't have any advantage over Vaporeon as a water-type attacker. But with Water Gun/Hydro Cannon, it will out-DPS Vaporeon by quite a bit. It might even become the highest DPS pure-water type attacker (though Gyarados' partial flying typing doesn't matter in most matchups).
Answers
Water is already the type that defines the metagame and Gyarados is predicted to get Waterfall. Buffing Blastoise, Feraligatr and Swampert (to make them viable alternatives to Vaporeon) would make Water, as a type, too dominant. Would rather introduce better moves for underpowered types, like Poison, Bug, Steel and Fairy.
