http://www.wowhead.com/item=10506/deepdive-helmet
In WoW, there
are items and equipment that enhance your underwater abilities, there are even underwater mounts. Basically you want it more WoW-like. :P
I really don't see how allowing a character to swim underwater is so.... radical to you. Making my character equip snorkels or scuba diving gear seems like more of a step away from Final Fantasy to me. I'm not saying there should be none, but I am saying you should have some faith in SE, because I am sure they can implement swimming and underwater content in a way that
most (not all, because no matter what someone is going to cry and bitch and moan) people can enjoy.
I remember Nyzul Undersea Ruins and I remember looking out the giant glass windows, wishing I could be outside looking in, so I could see what they actually looked like. I want something like that in FFXIV, I think it would be awesome to swim down and see some massive ancient ruins or some giant creature.