I am no expert in this field, but I am a little surprised. I thought that the Jabscos did not allow free flow of water through the pump like the aerator style pumps do. However, since trhat seems to be what you are getting, I think you are right that you need a shut off valve. If it were me, I would do both; a ball valve and an electric gate valve. If at all possible I would relocate the pumps to create space for a ball valve immediately inside the transom (for safety). Then, for convenience, I would install an electric gate valve between the pump and the bags. It should be a "normally closed" valve, that opens only when current is applied. Wire it to open whenever the pump is on. The best deal I found on these types of valves was from a company called STC. They were about $60 each. I have had 2 Jabsco Ballast puppies, both mounted horizontally. One was actually mounted horizontally, but upside down. They both worked great in this configuration. I'll do a quick search again on those valves...