Best Water Pokemon?
We all know Kyogre is the best (till we know about Palkia) but what of the rest?
Gyarados is solid with WF/HP but is ironically weak to Rock which it is supposed to counter
Milotic is not a very good attacker and not very easy to obtain (discounting the latest research quest) but is bulky and defends well
Vaporeon is plentiful, cheap to evolve and an amazing budget option but is not guaranteed because its a 33% chance after the 1st try
Which to work on?
Answers
So you want a water pokémon to counter a rock pokémon with good DPS but also good TDO?
Think no further than Swampert. He is already good with WG/S, but when it gets Hydro Cannon he will be even better.
Non water options include Groudon (but I suggest you would rule him out just as well as Kyogre), FP Venusaur, MM Metagross and more.
More of wanting a water pokemon to counter a rock pokemon, like it should
Then again between Groundon and MM Metagross I think that sector is covered
Guess its Garys for me to vs Fire pokes, but then again Rock has the exact same profile with both dealing super effective damage and having resistance, and SD Tyranitar does better DPS and TDO if I'm reading the spreadsheet right
Water pokes don't really seem to have meta relevance anymore though Gary bucks the trend by being as good as Grass pokes vs Ground because of resistance
Easily it’s gyarados as above. Not many have resources to max 6 kyogre (I certainly don’t) so gyarados is the best fallback.
You mention about the rock weakness which is true but it also bring to it immunity tier resistance to ground which pulls it ahead of kyogre for total damage output vs a ground target using ground moves. Flying typing is also a blessing because a few things gyarados counters (ninetales, groudon) also know solar beam and this again elevates it to just a tad below kyogre.
However with some CD water starters incoming i wouldn’t max more than one gyarados
What to work on. Save Totodiles for the CD in January and Mudkips for the CD in July. Feraligatr will become the best non-legendary water type attacker with Hydro Cannon, and 6 months later it will get outclassed by Swampert getting Hydro Cannon.
If you need one right now, get Gyarados for optimal DPS or Vaporeon for ease of access. EDIT: And don't worry so much about Gary's rock weakness. Rock raid bosses frequently sport non-rock moves that Gary resists, especially ground.