It all depends on how long its been in the water...
I say incubate the eggs. There is always a chance that it can still hatch. It doesnt hurt to take a chance incubating them.
One time I got 15 diamondback terrapin eggs laid in the water. Stuck all of them in the incubator and crossed my fingers. I got 15 baby terrapins to hatch out of 15 eggs laid in the water.
Its also happened to me with other species of turtle.
I'd suggest incubate them if you want a shot at hatching out some babies.
Also, if its only one egg so far, she should be laying several more, so keep an eye out for more eggs.