Scoop out the water the best you can, place your brick or rock on the bottom, position the post in the hole, do all this quickly before the water fills back in, then secure the post, then pour in a bag of "Fast Set" concrete and do not add water, the water table will mix with the concrete and make your set. Fast Set does not require any mixing, just pour it in dry and in your case the water will make the mix.
Sets in 1 hour or less.
Or pour in the fast set with the post in and the water table full and the displacement of the fast set will cause the water to pour out. Be sure to make your top cones from concrete to shed the water away from the posts.