Some information about the vessel you're planning the install and the make/model of the
heating unit would improve the quality of any answers .
As a general
rule of thumb I would expect to have a
diesel heater flued high up on the
hull or through the
deck. Clearly too there's a dual purpose to consider, 1. expelling the gases and 2. preventing sea
water ingress from waves and heel.
As the heater burns it sucks in air for combustion, obviously causing the
exhaust gases to heat to rise in the process. Those gases are obviously also toxic and we don't want them in our boat. So typically that requires a flue. And the hot gases in the chimney pulls fresh air into the heater as it rises up the chimney and exits.
Perhaps it is possible to run a flue to your engine's exhaust, but that implies getting those hot gases down low enough, and I would have through too. In order for such a system to function, the
engine would need to be running, Otherwise how would the pressure be sufficient to force the gas out through the exhaust system's water lock?