Water Desalinization

Since seawater is denser than freshwater, you could try installing some sort of waterproof barrier around your well extending maybe three or four meters below the high tide level. This should keep the seawater out of the shallower part of the well.

Oh, one might try media filtration below 5μm, carbon filtration to take out any taste, followed by a small reverse osmosis unit, then re-harden with finely-divided calcium carbonate and chlorinate.

Or one might try media filtration below 5μm, carbon filtration to take out any taste, followed by blending with the town mains supply to achieve a low-enough TDS and a zap with some UV prior to drinking it.

Or one might use the water without any treatment for the dozens of things that one does with water that don't require desalination (water the garden, flush toilets, take showers and baths, etc.) and rely on the mains supply for those that do.

The possibilities are manifold. Local circumstances will direct what to do, and they cannot be seen from here.