I take it the top of your riser is well above the waterline? Do you have a vented loop on the water intake line? If so, where?

Your hull is similar to mine. What general shape does your hose to the stern outlet take? Did you get a high loop before the outlet?

The top of the riser is above the waterline but not by a lot. In an ideal world, I wish the waterlock could be mounted a few inches lower. The vented loop is mounted on the other side of that bulkhead close to the bridgedeck. As per Nigel Calder, I have a hose barb at the loop top and 1/4" hose routed to a cockpit drain where I can easily see if water is pumping rather than using the standard failure prone anti-siphon valve. The exhaust hose follows the hull and exits at the same location as the previous copper exhaust, under the counter near the transom. The hose makes a loop tight to the underside of the deck just ahead of the exit fitting.