It seems they are all GM style 1 bar sensors but I can't validate that. Are the Mercury "marinized" somewhow???Are MAP sensors the same as the auto ones? I did see that NAPA has one for a fraction of the Mercruiser one, even lists it as a direct fit for my Mercruiser.
Also - do mean put some small wires in the back of the plug and a digital meter to check with the power on but not running?