Did this problem just start happening to what used to be a properly working head or is this a new installation?
Not so simple as having a anti-siphon loop. It must be installed at the proper height IF the head is below waterline. I will ass/u/me it is below WL because of the problem described. There also needs to be a solenoid on the vent that closes when the pump runs or else the seawater side of the pump will only suck air when running.