My hope is that they make swimming a "side" type feature and not the basis for an expansion like flying was for Heavensward. They could make some nice highly rare side quest and items gathering by using this feature and you would really have to explore to find these spots to go.
1. You could swim down to find a secret underwater dungeon.
2. Swim down to find places to mine/gather mats.
3. Swim down to find Rare NM or HNM to fight and get loot/gear.
4. Swim to find other things...possibilities are almost endless.