Not to digress from the original post but this idea could tie into, is a way that you could adjust an areas xp output - not just the mobs individually. So Aryth ocean which could use some love in the mob dependent, could get a bonus because it's right in the path of everyone, so that risk=bonus xp. But other areas could see some similar love like ugruks comes to mind.

Now one important thought I'd want to put forward is that this shouldn't be all about water shifters and Mariners from their point of view. It should be areas that appeal to all but can serve as areas in which a water shifter or Mariner could find pk prey too.

I've wanted to do a true Mariner for some time but your only real hope of catching most anyone is a quick pass across the Aryth... Not a lot of hero levels where you'll get to find anyone spending long periods of time in a water region