Wash / Livewell pump - unwanted water flow

[abl1111]

The set up is plumbed 100% right; valves and all. When the valve is open, which is the way I like to leave it, and I'm under way - the saltwater is being shot into the intake scoop, past the open valve, and through the pump at an enormous rate. There is a cap that 'caps' where a hose would normally be attached to. That cap is not doing anything - the water is blasting by it.

A spigot/gate valve thru the stern wall is the only idea I have thought of to reduce this. But, even though this will prevent water from blasting thru - will this pressurized water do any damage to the pump ( cause it's still getting to that point ) Capice ?