Until CB4 underwater exploration in ArcheAge was pretty pointless. They added treasure hunting though, making it more wprth while to do. So get your scuba diving gear and your balloons ready so you can get that treasure and float it back up to your ship!
Here is the official video for the underwater gameplay.
And here's a more in depth, unofficial video of the gameplay.
Pretty fun looking, right? I wouldn't care if they hadn't added this feature because I would have still gone exploring underwater either way.