The Guild Wars 2 site has been updated with a brief look at forthcoming changes to underwater combat. Originally designed to be "realistic and where fighting was slower", the team determined that some of those foundations needed to go and were ready to "rebuild something better".
To that end, the team has design goals for underwater combat to make it feel as effective than land-based combat. In addition, movement underwater should be more effective and skills should have a rough parallel to their land-based counterparts.
Your character may still appear to move more slowly than they do out of water, but enemies shouldn’t take longer to kill. Traversing an underwater environment should be a fluid experience, and you should feel like you’re still playing your own character and profession.
Most interestingly, however, it seems that there are nuggets of surprise and treasures to be found for those intrepid swimmers who take on the challenge of exploring the depths. "Compared to Tyria’s terra firma, the water is largely unexplored. Rumor has it that a dive master with a passion for discovering underwater treasure is arriving in Lion’s Arch, and she has tasks for Tyrians who are unafraid to take a plunge."
Read the full report on the Guild Wars 2 site.