Will as a Mermaid...

This picture shows Carolin meeting Will who was caught by her Endermen who were looking near a beach

This is Will's eye which is blue with tear shaped pupils.

Abilities:

Hydrokinesis

Shapeshifting but with only aquatic forms

Night vision to see through the ocean

Sensing emotions

Swift swimming

Reasons for abilities:

Got the abilities from The Illusionist who has aquatic mutations in this AU