How can I fill a void in a concrete seawall?
I believe there are cement products that will cure in water, however I would wait until the winter. Cold is not a problem here, except for your fingers. And you can always find a day in the middle of winter when the temperature is quite moderate.

Find a high strength patch filler. Make sure that it attaches solidly to the existing hole. If possible, I would drill some holes into the sides of this hole, then put some non-rusting (brass, aluminum, etc.) bar stock into the hole so that it sticks out into the void area. The idea is to put a set of fingers into the cement patch area to prevent this patch from popping out.