Well, 1 Agility doubles Spe, just like Swift Swim does, so any Pokemon slower than Cramorant is still slower if they both have their boosts.
PU: Drednaw and Carracosta are too slow, slower than Cramorant at least. Armaldo is weak to anything Rain boosted and does not get STAB for water moves, whereas Cramorant has STAB and resists water moves, whether they are used or not. Golduck is another option, but Calm Mind is its only move for boosting SpA, and it has no Speed boost other than Swift Swim.
ZU: Ludicolo, Poliwrath and Qwilfish are untiered for a reason, they're just not as good. Agility + Life Orb gives 2x Spe and 1.3x SpA as well.