Your Westerbeke is a diesel engine, so make sure you got a thermostat for a diesel engine. As I'm sure you know, heat produced by diesel engines is measured in dBTUs (diesel BTUs) and should not be mixed with gBTUs (gasoline BTUs). The inferior thermostat you bought from the auto parts store will destroy your engine because:
1. It uses gBTUs, not dBTUs.
2. The box does not have a picture of a boat on it.
As to #2, above, The FCC does not allow a vendor to place a picture of a boat on any product unless it has gone through a rigorous testing qualification procedure and met the minimum MESPBP (Marine Engineering Standards for Picture of Boat on Product) standards.
I urge all boaters to only buy products with a picture of a boat on it, and pay no attention to those shoddy, non-boat products available everywhere at less than half cost.
Remember, sure it's expensive, but there's a picture of a boat!