45 years in water
“Stainless steel” covers a lot of different alloys. 316 is the most commonly known, but not the usual choice for most manufacturers when the application involves contact with seawater. Duplex seems to the popular choice these days.
But calling your high pressure pump you “boost pump” is what jumps out at me. The configuration you describe seem to be pulling water
through your pretreatment filters rather than boosting pressure from from a feed pump.
Suggest you get a small pump to feed the filters, which will make you high pressure pump a “boost pump”.
These high pressure pumps never like to run without a well flooded suction. Industrial applications require a positive pressure at the booster pump suction before they begin ramping up.