I am the "if it looks stupid but it works it's not stupid"- kinda guy so first of I don't know anything of how your project is set up but I would make an effect something like;
- Put the water on a deifferent layer underneath the grass tiles and everything else.
- Have the water extend out from it's parameter (because I guess you would like the reflection to start not just when the character is at the edge of the water)
- Check if the character is overlapping the water object
- If so duplicate the character, but mirrored and flipped (i assume the image point is at the bottom of the characters feet)
- and change the oppacity...
Then If you no longer overlap with the water destroy the duplicate character...
Now I haven't tested this and there might be waaaaay better solutions but something along those lines would be my firt attempt