I use the head sink drain as a source of fresh water for flushing the head. I added a Y diverter valve to the head inlet hose. In one direction the valve connects toward the existing head inlet thru hull, and in the other direction it connects toward the sink drain hose. Also added a one-way fitting to the sink drain hose just above the thru-hull so lake water can’t be sucked in when flushing. In fresh water mode I turn on the sink faucet, let the sink fill a bit, and then flush the head. If I want to use lake water (or salt water in your case) turn the valve in the other direction. Works well.
We fresh water flush on my 387 but didn’t change any plumbing at all. Simply close the seawater intake through-hull so you don’t suck in saltwater