Unfortunately there's no canon word for "water." There is the invented word lom but I would really not recommend getting a non-canon word tattoed. You could use the canon word okaaz "sea."

Here are the runes for each of these:

Gol "Earth"

Yol "Fire"

Su "Air"

Okaaz "Sea"

Here's a list of the alphabet. You can download the font yourself here. The "aa" in okaaz is mapped to 1.