you are weak to water and will take double damage from it

if you start your turn in water you take 2d6 cold damage

Once per long rest you can burn up, for 5 turns.
your melee attacks deal 1d6 fire damage

things within 5 ft take 1d6 fire damage on their turn

while burning up if you take damage from water the effect ends right away

due to being from the fire plane you have become immune to all kinds of fire damage.