Scuba diving involves the use of a tank of compressed air, attached to a first stage which reduces the air supply pressure to the pressure of the water surrounding you, so it is breathable. A regulator goes into your mouth (mmm tasty salt flavour) which you breathe from. The kit is somewhat cumbersome although its not bad, and breathing from nitrogen-enriched air (Nitrox) gives you a good buzz.
Shaman have the ace underwater breathing spell, which lets them breathe for 10 minutes without anything, save a Shiny Fish scale as reagent. It can then be recast any time while underwater, so effectively you have infinite time.
Underwater WoW is a bit boring really; we have a giant pump in Zangermarsh and some waterfalls, a few mobs of various types. There are some ok shipwrecks, but these are usually murloc infested, so unsuitable for exploration. Unless you like killing murlocs. And who doesn't?
Scuba divers have far more interesting places to dive, given the world is somewhat larger than the world of warcraft. We have reefs, fish habitats, really massive shipwrecks, dropoffs which descend 100s of metres to the sea bottom, the list goes on.
Getting to the dive site
Shaman have water walking and probably a flying mount, allowing us to skip most obstacles such as underwater mobs and land mobs and drop right into where we are diving. For travelling across land, a flying mount is better than a beaten up dive truck any day. Walking across the water then dropping in is far superior to struggling up and down a sandy beach with several tons of dive gear on.
Scuba divers usually either have a beaten up dive truck if shore diving, or a boat if on the sea. A boat can have drink, beer and food, so this definitely beats a flying mount on which you can't even drink or eat. You can also snooze on a boat between dives, try that on a taxi and you'll discover yourself logged out.
A Shaman has mucho defense, lightning, fire, earth shocks. Unfortunately the underwater habitat of WoW is a little more hazardous than the world.
A Diver, well will have a small knife if they are lucky, if they are not in national parks or places which prohibit such things.
Shaman get to tap up the many Mineral nodes we can find underwater. We can also kill mobs for various vendorable items and RRQ items, for example Unidentified Plant Parts in Zangermarsh.
Scuba divers on the other hand get to keep sea shells, if the laws in the country allow. There maybe treasure but generally such things are protected so no steally!
Ok so diving in the real world definitely beats diving in WoW, even with a Shaman which seems to be the master of underwater activity - apart from those druids and their pesky Seal form. It is a pity there isn't more interesting and cool underwater architecture in WoW, but I guess the designers probably thought people would see it once, die when they ran out of breath and never come back.