To drain the antifreeze completely from the HW heater you must loosen the hose clamps at the water heater, then press on the schrader valve to let air in to help drain by gravity. Another way is to disconnect both hoses at the engine and blow air thru one and the antifreeze will come out of the other. The purpose of the schrader valve is also to vent the air to prevent airlock when you refill the engine with antifreeze. Most engines have a typical radiator drain instead of what you have. The schrader valve must have been installed by a previous owner or is a new Beneteau feature. Good luck.
I did not want to make a mess by the water heater as it would be difficult to drain the heater and contain the anti freeze. Using the hoses at the engine, I blew out the old antifreeze. Quite a bit of sentiment also came out. I filled the hose several times with fresh water and blew that out to clean the rest of the sentiment out of the heater and its hoses. I initially refilled the antifreeze through one hose at the engine using a funnel. I tried multiple times over several trips to bleed the air out of the heater hoses using the schrader valve, but could not get the antifreeze to circulate. I connected a hand pump bilge pump to one of the hoses to draw the antifreeze in through the other hose and to also pull the air out of the hoses. This worked and the antifreeze is circulating. There did not seem to be much air in the hoses,but apparently the engine water pump can't purge any air out of these lines.