most of the water heaters which have engine antifreeze loops run the hot engine antifreeze through a stainless steel tube inside the water heater. check with Seaward to make sure this is the case for yours. the hot antifreeze from the engine, running through the stainless tube, then heats up the water in the water heater. so you can run the engine with zero water in the water heater, and do nothing to the engine hoses into and out of the water heater. even if the water heater body fails and leaks, the stainless loop for the engine hoses remains intact. if you s tart needing to top up engine antifreeze , make sure it's not leaking from inside the HW heater loop. then you will wish you had the bypass already installed so you can run your engine.
it's nice to have a way to bypass the hot water heater if you need to work on it. if you are replacing all hoses anyway, why not put in a bypass? usual the engine hoses are 1/2 inch hi temp , you can buy NPT hose barb brass fittings and rig your own bypass for about $8. (DON'T use plastic fittings , heat of the antifreeze will damage them.)
