Gastro and Jellicent fine, Cloyster I guess since it enjoys having Rain to boost Shell Smash.
Lapras has a niche value in that it has access to Boosting move + Hydration. Empoleon im a little bit uncertain of. O.k it checks kingdra, great, however Ferrothorn checks Kingdra too. Also, why Kingdra might be the best possible threat to Rain its not commonly used enough for me to consider Empoleon. Empoleon just seems kinda "meh" when you factor in that Ferrothorn can do that job better, while still giving an electric resist.
Slowbro might need more love but I don't think the Rain Article is the place to do it. I just don't see what Slowbro brings to a Rain team other than "its STAB attacks get boosted" and pretty much every other water type can claim that. I think Slowbros biggest problem is on a Rain team, ideally, you want to keep your water types at a minimum to reduce the chances of something like Thundurus or Rotom W from blasting right through you. With this in mind, would you rather have Slowbro, or Gliscor who walls similar things, can stall with Sub + Protect, stallbreak with Taunt or threaten a sweep with Swords Dance. Gliscor also does a much better job against LO Admant Excadrill, (where as Slowbro gets OHKOed by a +2 EQ with a little prior damage) AND can lure out and dispose of Rotom W via its SD + Flight Gem + Acrobat set. To my mind, i will always take Gliscor over Slowbro, even on a Rain team.
EDIT
nvm Gastrodon is not getting a mention in the abusers section. It does not really "abuse rain" as well as others (yes lapras is on there but it actually has a niche value. Besides, it is more put on Rain teams for defensive purposes against opposing Rain teams . That said, it will get a mention in the Rain threatlist which I think is satisfactory.