This issue was brought up by

Whats bad about water and underwater combat in MMO’s? This was likely eluded to from the recently released game play videos for Guild wars 2.

I picked a few key quotes, have a look:

Nils said: There are some things where I have trained myself to suspend disbelief. I don’t want to need to take a piss every now and then. Really.

But giant fireballs 2000m below the ocean’s surface? Warriors that happily jump into the salty water with their plate mail? Rogues that don’t fear about their leather armor? And do you know what it feels like to stay in salty very cold water for that long? The pressure ?

Syl said: @Nils
“When did you last see it in real life?”

Oh come on, seriously that question from you? 😉 if we tried answering this consistently, we’d have to erase at least 50% of the features that make MMO worlds magical and fun.
to me there’s no immersion loss here; just like I love to fly on a gryphon or hurl firebolts, I love exploring underwater themed zones. it’s part of the magically enhanced “Me”. ^^ but I guess it depends on whether you prefer the ‘medieval/historic’ MMO approach over the epic fantasy one or not..

All who weighed in on the comments were right, the downsides are phenomenal, the upsides are marginal, and the realism vs. fantasy doesn’t work for most (some like realism, some like fantasy). If you embrace the fantasy, you alienate those who prefer realism and vice-versa.

The reason I believe that games embrace water is because 90% of any fantasy based MMO’s takes into consideration the realism of the elements. Earth, wind, water, fire, life, death etc. With this said it would be illogical to believe that creatures of the water could survive on a planet (world) with no water. Furthermore there would be no way to justify the reasons for said creatures to ever come up to the surface unless they have been in some way or another effected by those who live on the land. There lies fantasy vs reality, and why water is a part of the game and a mechanic we will likely be dealing with forever. Mixing magic with knowledge was the actual bridge to science (real world applications of science) and this is how fantasy survives.

On another note, the feeling of the “unknown” is a big deal for MMO’s and some people approach a  world of fantasy with a certain amount of real fear. This is what the development team hopes for and designs the entire surrounding on the fact that it evokes some sort of emotional reaction from you the player.

My personal opinion? I like it when it fits the bill, and sometimes it just annoying. The Z axis can be a little overwhelming, on the flip side, I can’t tell you how many times I’ve realized I was in trouble (PvP wise in open world WoW) and went running for the water to get away from the harasser (I play a druid). I like the water zone in cataclysm to a point but I have NEVER finished it.  It can get discombobulating and the distance checks are a bit of a disaster when playing a ranged class since there are no real reference points but, all in all most developers either nail it or destroy it in terms of fun.

General opinions vary, but for the most part, your either for it or against it. The realistic view of things is that game worlds will continue to develop underwater worlds for us to explore, and we will either deal with it or find another medium to get enjoyment out of.