Gills
There needs to be a Gills Evo that is unlocked by fin. It should swap all land tiles with water tiles and the next time you enter a body of water Gills activates
you will now take damage outside of water as a status effect like rain heat and cold (unless it's raining)
Damage and speed will go up by 5% when in water
and water terrain adaptation will + 10-20% and land terrain adaptation will go down by - 10% but slither will remain but only in water