I've built aircraft fuel tanks (and whole aircraft) out of fiberglass and am certain that it will work for water - even nasty black water. That said, it is not an easy or inexpensive solution. Unless you need a very unique shaped tank, I would think that an off the shelf plastic tank would be much less expensive.
There are several approaches in addition to the lined box suggestion above. Making a mold out of foam, sealing it, glassing it, and then removing the foam is another. Glassing both sides of 1/4"-ish foam and then using those panels to build a box (and glassing them all together is another approach. I can supply more info/links if you decide to go this route.