I don't know if I have the same unit as you but my Vetus exhaust muffler is about 18"above the water line in the transom. My engine is completely below the water water line.
Since my engine is about fourteen feet from the transom I have custom built exhaust elbow
(machine shop) and water collector that box I built myself since the Vetus one would not fit in the bilge blow the cabin sole
. When the engine shuts down there is about 1.5 gallons exhaust water that back flows into the collector box which can hold over two gallons. The collector box has a valve to allow it to drain the back flow into the bilge.
Under sail with the engine off this valve can be left open direct any stern wave action from being pushed into the system and maybe make it up the exhaust elbow and manifold. This is highly unlikely. except in sever conditions. since the muffler at the transom is nearly four feet above the water line.
Down stream of the elbow the raw water
is sent to a conventual U-shaped anti-siphon unit. I threw away the little bleed valve that is provide since they usually clog up and ran a small diameter hose from there to one of the cockpit
drains. A peek into the cockpit
drain when the engine is running will tell me if the anti-siphon unit is doing its job.