Water...oh sweet life giving water..
Moving on from whatever that intro is, water is a very very important typing in rby. From starmie and cloyster holding OU together to the excellence of lapras in UU and even directly upstairs with mons like seadra and poliwrath in NU.
It stands to reason that water is probably pretty important in
PU right?
ABSOLUTELY.
in this post i plan to explain what exactly the PU waters do, and just how excellent they really are.

with the removal of rapidash, golduck stands tall as probably the defacto #1 pokemon in the tier and its not hard to see why. Golduck possesses an extremely important speed tier, warping the entire tier around itself. Golduck is popular in the back for its great power and ability to check


etc. As well as sweep with amnesia under the right circumstances. Golduck slowly has also caught on as a lead, using less typical moves for it such as hyper beam and counter in this slot to better take advantage of its competition. Golduck sets a gold standard for water types in RBY PU.

Seaking as been a potent sweeper for a very long time in RBY PU. Despite how long its performed its role its moveset hasn't been altered much, typically opting for a simple surf/blizzard/agility/hyper beam cleaner set. This set takes advantage of all of seakings strengths as a late game water type, using its powerful stab to break holes in enemy's. Blizzard to ruin grass and flying types hoping to trade the mon down. Hyper beam is around mostly for other waters like

who famously can fall to a mere hyper beam crit.
Agility not only makes up for Seakings milling speed, but after an agility there is nothing that can revenge it without a serious beating.

taking a break from the powerful fully evolved water types, staryu comes into focus as more of a response to how good the other 2 are than anything. Being a water type with access to recovery and Thunderbolt it serves as a sturdy check to seaking(ignoring crits) and a useful answer to golduck, albeit already boosted golduck can be troublesome. Staryu is not only here for waters however, its excellent at spreading paralysis for its allies to break and it checks the same mons the other fires aim to in a pinch.

a hefty drop in usage from the other 3 admittedly but Slowpoke deserves its spot nonetheless. I have experimented with this pokemon as a lead a fair bit lately, even getting this idea onto the sample teams. I believe this pokemon is excellent at exploiting common leads but its far from without issues as its venusaur MU is naturally horrendous and it doesn't appreciate the existence of lead staryu albeit rare. My preferred set was twave 3 attacks but u can try amnesia if you'd like.

even less common than Slowpoke we have poliwag, another water type lead hoping to take advantage of high lead

usage
and the high usage lead

's around to combat it. Poliwag is a simple mon, it clicks hypnosis and the other basic water type moves. Much like

in NU.
I haven't used this mon much myself but its seen fine results.
Beyond these pokemon you can probably find a use for

however i don't particularly recommend it.
These are just quick summaries of the water types in current PU.