Using 5.0.4 - 16016 (Live): I just went to the Deadmines test area with a few others and found the only way to successfully land in the water was using the Nitro Boost engineering tinker and being slowfalled by a mage (I imagine the parachute would work too). Didn't have anyone with Rocket Boots to test.
For the non-engineers we tried the Swiftness potion, Figurine - Golden Hare, Paladin Speed of Light Talent and several combinations of those. None allowed the person jumping/slowfalling to land in the water. When watching them it would seem they wouldn't make the distance or would be on an angle that would have them land below the 'ocean floor'. The others in the group were not experienced explorers so fine tuning was a bit hard to do but I thought I'd offer my experiences/info anyways.
Upon each failed attempt, we would fall, get d/c'd once or twice and upon staying in the game would be where we were, fall, then get a load screen and be ported to the start of the instance but were never ported to the started without being d/c'd at least once first.
When I went over to join the 1 other engineer I noticed that my 'landing-point' was above the 'ocean-floor' and I only had to swim up a small distance as opposed to the numerous times I've been there before, with nothing more than a jump and slowfall or levitate and had to swim up a lot more (falling so far also made the trip more exciting for new explorers).
While over there I went to walk under that little gap between the ground and the rocks surrounding the middle hole and was instantly d/c'd as soon as I was below floor level but to my surprise was still on that spot (1 step prior to being under) when I logged back in.
Also noted was the lack of the blue texture/graphic when standing beside/walking into a boundary wall.
After that I made an attempt to go to Dal Arena (with the NPCs) but was thwarted by constant d/c's any time my zone changed to "Dalaran" although after reading these I do believe I was possibly in water, hard to tell as it changed audio/water effect just by turning the camera (I am familiar with the camera being in 1 type of area while my char is in another but was still uncertain). Admittedly my aim was a bit off since it's been a while and subsequent attempt while dodging the water my yield the desired result but any/all information at the moment could be of use.
Will try for Kara crypts/smiley in the not too distant future and post back with any useful results.