Could you pump just a bit of seawater to augment what water you have? Mildly brackish water could be tolerable for some of your fish species?
As FishinRod noted, the water level of the pond would still seek equilibrium with the present water table.
Back some time ago we had a forum member with a saltwater pond. He had tarpon, maybe snook, and other fish that can move from ocean to brackish water. I don't recall if he also had what are traditionally considered fresh water species such as bass, tilapia, etc.
It could be that you are in a nearly unique situation to have both fresh and salt water fish.
Just a thought.
