You talking outboard or inboard? I'm assuming it's an inboard 'cuz you said "engine".
I spent half my adult life selling parts, so I'm pretty good at cross-referencing and outsmarting the OEM guys. e.g. I needed an intake manifold gasket for my Mercruiser, and they wanted $85 for it. It was metal, unlike the automotive ones. Couldn't find one on the aftermarket. So I did some playing around with the Mercruiser part # and found a GM number. Sure enough, there it was, for less than $30 at a GM dealer. Yee haa!
Most inboard parts are the same as automotive, except for the USCG approved stuff and things like exhaust gaskets and cooling systems. I can usually find everything I need on Amazon. I live in a small town and there isn't a lot available locally unless it's really common.
Once you know the part numbers you can buy anywhere.
Good advice. Save your part numbers, especially for things you need repeatedly. Eliminates confusion and saves a lot of time researching crap.
And I wouldn't buy anything at West Marine unless money is no object.