The DrDaq would be most suitable for your application. You will be able to plug the pH probe directly into the unit and set an alarm condition so if the pH probe goes out of range. You could set the digital output of the DrDaq to switch your solenoid circuit on or off.
The pH proble should be ok in sea water. You may find that the probe will deteriate after long periods in situ, and will require regular re-calibration. Re-calibration requires two buffer solutions, exact instructions on how to do this can be found in the DrDaq manual