User grows gills.

Pros:
-user can breathe underwater
-user is well adapted to swimming
-user is athletic most likely
-user is naturally slim to glide through water

Cons:
-user must drink a lot of water to prevent dry out
-user must swim for more than 10 hours each week
-user must keep gills uncovered

Weakness: dehydration
User will die and their skin will dry up.