Hello: “Salted” water sounds quite conductive. A glass lined tank type heater won’t stand up to highly conductive water. A stainless or otherwise non-ferrous oil fired boiler might be the way to go if you already have oil. If you already have a large enough oil fired heater for domestic use, you could plumb in a heat exchanger to use some of that heat for the pool. I think heat loss calculations are in order so that equipment isn’t seriously undersized. A pool cover hould have a huge effect. Hope that is of some use.