I don't know that there is a standard location for this as it can be installed by the broker, contractor, or owner along the way. My boat had a breaker badly installed in the battery compartment, many others I have seen have an in-line fuse installed in the battery compartment, I have changed mine to a labelled breaker on the eletrical panel.

The manufacturer typically recommends that you give the heater power directly from the batteries, rather than from a bus on the panel, so most often an inline fuse of some sort gets used I think.

Do you guys appreciate the reason for the inline fuse and NOT having it on a breaker? This is to prevent overtemping the heater.

Assume that the heater is going full blast, and you decide to switch off the power to it at a circuit breaker, instead of turning it off at the thermostat and letting it power down according to its cool-down program. What happens? Immediately the fan in the heater will stop, and the burner can inside the Espar will not be able to cool itself.

This furnace is analogous to a small jet engine on an aircraft. And jet engines (turboprop or pure jet engines, they're the same) are cooled by the air going THROUGH them, not by the air flowing AROUND them as in a piston airplane engine. It's important for the life of the furnace (and possibly for the safety of the boat - ie, to prevent a fire internal to the furnace???) that it ALWAYS be shut down with cooling applied.