Our boat was a fresh/brackish water boat too, until we bought her 3 years ago. This spring, after I replaced the raw-water cooled exhaust manifold and a host of other parts that needed replacing after 40 years, I wanted to have the capability of flushing the engine. I used a swimming pool 3-way valve (it's tough and built to withstand caustic pool chemicals so salt water isn't an issue for it), a couple of fittings , a piece of scrap garden hose & a bucket. When we're done using the boat, I open the hatch, move the valve from the seawater strainer position to the flush position and suck 5-10 gallons of fresh water through the system. When I'm finished, I switch the valve back to the the seawater position & we're ready for the next outing. I sleep better knowing the engine is full of less corrosive fresh water when it sits idle for any length of time. It'll come in handy when it's time to winterize, too.