Be VERY careful during <a href='http://www.wowhead.com/search?q=swift+action' target='_blank' rel='nofollow'>Swift Action</a>.
If you start it while you still have a Breath Meter, it treats it as if you don't have the Sea Legs buff, but rather are simply underwater, which can and WILL kill you during the course of the trip.
Oh, and if you're like me, you won't find your body afterward either, so you might be taking 10 minute rez sickness like I did.
And no, the Leave Vehicle Button does not work.
Trust me, the instant I noticed the breath meter going down, I was hammering that button.
I still managed to finish the quest in the first go, but at least half of the tougher mobs had been "tapped", meaning I wasn't getting credit for them. I probably would've passed it and NOT DIED as well, had so many not been tapped. <_<
The "You can't do that right now" was in response to me rapidly clicking the "Leave Vehicle" button.
I've already posted a thread on the Wow Bug Report forum about this, but I wanted you guys to know in case you or any friends are still running through there.
EDIT: I'd also note that the Horde side version of "Full Circle" suffers from the same problem, only the trip shouldn't be anywhere near long enough to kill you.
The Sea Legs Buff was even active, but did not stop this problem.
Oh, and I thought I'd also mention that in BOTH version of "Full Circle", when you get on the sub, it looks like there's only a tiny area for you to stand in, indicating that there might be an automatic elevator down. There is not.
Head left or right and you should find a tiny passage that has a passage down at the opposite side from the door.
Go down that and make your way down until you meet the captain of the sub and stay there. You
have to walk down and around into the sub proper and listen to almost all of the captain's speech if you want to get credit for the mission.
A lot of people have assumed there's no way down and haven't been getting the proper credit, and since you have to go back to the previous ship and ride down on the sub AGAIN to try again, you REALLY want to save yourself this trouble.
I should also note that both Alliance and Horde have a quest that's miss-able, though the Alliance are far less likely to miss it.
The quest, which starts upon taking down a specifically named NPC of the other faction, is "Secure Seabrush" for Horde and "Claim Korthun's End" for the Alliance, and that NPC counts towards the 7 you have to kill.
The thing is: the targets for the Alliance are far easier to find as they're at the top of the assigned questing area and around the quest mobs you're more likely to chase down. The Alliance Sea-Scouts are further down and if you don't run across one (
which is likely since the quests that would send you near that depth are going to send you further down and further west than they'll appear, and you HAVE to kill an Alliance Sea-Scout to even GET the mission), chances are you'll mission out on some tasty XP and money without even knowing it.