Level 100 Statistics (see level 5, 50, 100)
Whiscash is quite versatile for its typing and one of the better bulky Waters available in the NU metagame, but really should stay there. It is outclassed in pretty much every way by Swampert in OU and Quagsire in UU. Be prepared to see it on opposing NU teams though, and definitely consider it for your own team. As long as it stays away from Grasses, it can be quite difficult to take down in NU play.
Whiscash is the NU equivalent of its Water / Ground type cousins Swampert and Quagsire. High HP and decent defenses allow him to take hits relatively well for the NU environment. Mixed STAB attacks are always nice, and Ice Beam hits incoming Grasses and Fliers. Toxic is seen on this version over Surf or sometimes Ice Beam quite a bit. Rest is usable in NU because things don't hit as hard and you can actually get away with it. There are no real bells or whistles on this set, but it works quite well if you stick to NU. With the EVs given, you will always survive two Adamant Pidgeot Returns, and usually survive two Jolly Pidgeot Double-Edges, or one Plusle Hidden Power Grass.
More offensive set. Hidden Power Grass is preferable to surprise other Whiscash and for the extra 5 Base Power but Spark is usable if you fear Pelipper for some reason and like paralysis. Whiscash can make 101 HP Substitutes, which will frustrate Seismic Tossing Lickitung and Wigglytuff. Toxic is mostly because Whiscash really doesn't have much else to use in that slot, but can help against Wish Wigglytuff.
Attacking from the physical side is also an option, although probably not the best one. This is mainly to show Whiscash's versatility. Hidden Power Bug for the inevitable grass switch in and also hits Chimecho hard. Surf is useless on this set, as it will hit nothing harder than your other moves, so Sleep Talk is really just a filler, but helps against random Sleepers (watch out, though, as most carry Hidden Power Grass on at least some of their movesets).
Whiscash really doesn't have many other options. Tickle can be used to counter Cursing Lickitung. Protect for Leftovers recovery and to scout Choice Banders and Hidden Power Grass.
Whiscash's stats are pretty balanced and allow you to go in many different directions. For the standard, you'll want to EV to at least 401 HP and get your Defense around the 230-250 area. That amount of HP will allow you to survive one Modest Plusle Hidden Power Grass at full health, even if you don't EV in Special Defense. For the special and physical attacking variants, you'll want max (or near) in its respective attacking stat, spreading the rest in HP and Defense.
Checks and Counters
Anything with a Grass type move is an immediate threat, so watch out for random Hidden Power Grass.
Dewgong can come in on an underpowered Earthquake, Surf, or Ice Beam but shouldn't be a problem if you run closer to max attack or have a super effective move. Grasses will scare Whiscash off but can be hit by Ice Beam on the switch in and Roselia is neutral to Earthquake. Choice Band Pidgeot will typically do ~50% or more with Double-Edge, so try to avoid that. Chimecho can counter the mixed or special versions with immunity to Earthquake and Light Screen or Calm Mind.