Kingdra should be B- rank. I definitely agree that Haxorus doesn't "hopelessly outclass Kingdra" or however you guys put it, however it gives extreme competition to Kingdra on the physical end (I thought Kingdra's DD set was one of its best ones), something that imo limits Kingdra enough to drop it just a bit.
Kingdra is interesting. Don't bother with DD, Haxorus is straight better. Special sets, including CritDra, are largely inferior to Hydreigon. I dunno how good manual rain teams are in UU, but I can't see much of a reason to use Kingdra otherwise. Its not a bad Pokemon, just outclassed. B- seems better for it.
Haxorus doesn't have Swift Swim, meaning it can't boost its speed by +2 in one turn (as well as water STAB, which it doesn't have.) At +2 speed Kingdra is faster than every scarfer, unlike +1 Haxorus which can be RKed by a number of them.
The only reason why you would compare Kingdra to Hydrei and Haxorus is cause they're all dragons... but Kingdra functions differently than them. Hydrei is a mixed/special attacker, Haxorus is a set-up physical attacker but Kingdra is a rain sweeper, a role which is has next to zero competition in and a role in which it's really effective at. And no, full-rain teams are not very good but you only need to pair it with Tornadus or something. It's a ridiculously good sweeper that just plows through most offensive teams.
When I see B- I see stuff like Machamp, Mega Obama, Durant... just these things that are honestly only sort of okay in UU. Kingdra is not outclassed, at all, in its best role (Swift Swim) and is just more effective than stuff in B- by a longshot.