Okay, I've tested Seadra thoroughly and I've been thoroughly impressed. Swellow can't even 2HKO the physically defensive one with Brave Bird and gets destroyed by Ice Beam, whilst Adamant Scarf Sawk can even fail to 3HKO with Close Combat if you're lucky, and gets wrecked by Scald. CB Rapidash can't do shit, even Wild Charge fails to 2HKO. Stuff like Regirock is also destroyed, whilst it could deal more effectively with Alomomola.
I've been using Scald / Ice Beam / Toxic / Rest with Aromatherapy support from Vileplume, and as a straight-up physically bulky Water it's been very impressive overall. And when I say stuff gets wrecked, it actually does. Whilst Wartortle and Alomomola can fail to OHKO even these frail opponents, Seadra has much fewer problems as it can score many OHKOs in offensive opponents with SR, and the inevitable residual damage from Life Orbs and recoil moves, and can thus make a better bulky Water on offensively-orientated teams that don't need Wartortle and Alomomola dragging their momentum down.
Using Disable over Toxic also gives it the niche of beating any CB attacker below 206 Speed, and many above it.