If there is a coral base and constant contamination by sea water, no such thing as a long term improvement. Best improvement is obtained by stripping off at least 18" of the imported dirt and replacing it with a low CEC media. That's right, I do not want it retaining Mg or Na. I would rather supply what the plants need on a regular basis.
They who can give up essential liberty to obtain a little temporary safety, deserve neither liberty nor safety.
Benjamin Franklin 1775
Sell not virtue to purchase wealth, nor Liberty to purchase power.
Benjamin Franklin Poor Richard's Almanac1738