One thing I noticed about NU is the great amount of good water-types. A small comparative list:
-Seismitoad. Great Pokemon, can set up SR and stops other water types such as Lanturn in their tracks. Has a decent movepool with Knock off and Sludge Wave. Watch out for HP Grasses on things like Magmortar.
-Qwilfish. Also, a great counter to many physical pokemon. Like Seismitoad, has a great movepool with Taunt, Thunder Wave, Spikes, Toxic Spikes, etc. Decently fast for a defensive pokemon. Poison is a great secondary typing which ensures Qwilfish resists Durant's X-Scissors, Virizion's Leaf Blades, Sawk's Close Combat etc. Like Seismitoad, doesn't have reliable recovery.
-Lanturn. Again, great typing and good bulk. Lanturns niche is Heal Bell, which is really useful for balanced and defensive teams. Volt Switch is another obligatory move. Doesn't mind HP Grasses too much and is thus a very reliable check to most fire types.
-Carracosta. More fit for Shell Smash. Water/Rock isn't the best offensive coverage, but it does the job. Respectable defenses + Sturdy often ensure a Shell Smash.
-Barbaracle. Should only be compared to Carracosta and Omastar. Faster and more powerful, but has to rely on inaccurate moves, plus doesn't get access to Aqua Jet which Carracosta does. It does get Fighting coverage though.
-Mantine. Great as a purely defensive pokemon with Defog, comparable to Pellipper. Doesn't get Roost or U-Turn like its Water/Flying companion, but has Haze and way better Special Defense to make up for it. Counters Typhlosion to hell and back. Also, Grass Types are not really an issue with Air Slash.
-Ludicolo. Unique Typing, rips holes in teams with a LO Rain Dance set. Also has good coverage with Hydro/Ice Beam/Giga Drain. No real negatives, except that you'll need to find a turn to set up Rain.
-Samurott. Can run two viable sets with SD and Special LO. Megahorn is its niche over Feraligatr. Has decent stats but stands out in none. Often outclassed by things like Feraligatr or Ludicolo.
-Poliwrath. Another unique typing, good defensive pokemon that stops Durant. Circle Throw is a good phazing move and Poliwrath can run a few different sets, with Resttalk, Toxic or Bulk Up, or a combination of them.
-Basculin. Good CB user. Adaptability helps power up Aqua Jet and Waterfall, but is definitely not one of the best water types out there. A lot of things are capable of using Aqua Jet and doing more than that.
-Feraligatr. Good bulk, access to SD, Dragon Dance, Aqua Jet, Ice Punch, etc. Very good pokemon but lacks Speed when not running DD. Good late game cleaner, but isn't really popular on the ladder yet.
-Pellipper. Another defensive water type. As said, Roost + U-Turn + Defog and good Defense. Reliable pokemon for a core. Actually the only Water type in NU with reliable recovery (!).
-Gorebyss. Another SS user. However, can also pass the boosts to other pokemon sweeping in its place. Therefore a very dangerous pokemon. On itself it's often outclassed by the other Shell Smash users.
-Omastar. Similar to Carracosta and Barbaracle, even has the same typing, but attacks on the special side, thus having Water/Ice coverage rather than Water/Rock. Not much to say really. Also has access to Spikes and Toxic Spikes if you really want.
-Simipour. Fastest Water Type in NU with 101 Base Speed. Also has Nasty Plot, but is very frail. Most priority moves can deal with it. Also walled by other water-types such as Lanturn and Seismitoad.