Doing that Kosumoth attunement and that darned pet has been up a zillion times, but the time the mount was up I think I was at Blizzcon so I didn't do it. However, since it was up this week I told HK we must go do it. Xyn had told me it was quite easy and he wasn't wrong!
He tanked, I healed and it went down. Yay!
Ok, the annoying thing with these underwater mounts is that you still run out of air underwater when riding one of these things. I wish that would be fixed! If my head pops up too much above the water the mount gets despawned. Annoying!
But that makes mount #276. Ugh, so many to go before I get to 300! That is, if I could be bothered to get mounts :P