I don't know what OU metagame some of these people are playing, but OU will always be a "weatherwar" tier. It has been since Poli+Tales release, and will be they're deemed Uber, if they're deemed Uber. Adding Excadrill and Kelp won't be changing much. I can see Garchomp finally getting that retest he deserves and Thundurus, too, getting a retest, though, given he loves the rain, may be deemed Uber all over again. Excadrill probably won't be the best idea to drop down.
As far as I'm concerned the only hard "counter(s)" to Kelp are Latias, and to a lesser extent Latios and Toxicroak. Celebi is not doing much to Kelp, nor is Tenta/Jelli, so they're probably more likely to be checks rather than counters. I also appreciate the person that reminded some of the earlier posters what checks and counters are, as, we learned Starmie, Dragonite, Salamence, Tentacruel, Jellicent, Celebi, etc. are not counters.
Kelp will definitely be destroying near everything in rain. 2HKOing just about the entire metagame with Hydro Pump is a major accomplishment, a Sp-hitter eliminating Blissey without boosts, not to mention pretty amazing typing matched only by Poliwrath, who might as well not exist at all; I think it's safe to say Kelp will be considered one of the best sweepers of BW2's metagame.