Chris. install a concrete caison (pipe with no sealed ends, when installed, Do not pump out but allow water to find its own level, then hire a “Concrete Pump” & drop the end into the water so that the hose is verticle to the base, then pump or “Tremmie” a plug in the bottom, when concrete has gone off you can pump out the water & pour a new base. (you may flatten but do not disturb the submerged concrete or you will separate the concrete)
Make sure the mass of concrete exceeds the “Lift” of the new Pump Well.
Bob