My cousin works at a boat dealership in New York and he advised me to get an independent mechanic to inspect the boat on the trailer, perform a compression test, and check it out in the water prior to purchasing it (It is a used boat).

The dealership offered to do these things for free but my cousin cautioned me about this. He said make sure you use an independent mechanic.

The dealers down here didn't really think it was a big deal, maybe because the boat is still under warranty, but I have never owned a boat and I don't know how well the previous owner took care of it. I'd prefer to have to looked over by a professional who doesn't work for the dealership where I'm buying it.

Has anyone else had this same situation?

I definitely wouldn't trust them to do it.  It could be completely fine, but I'd rather have the piece of mind and the non-biased advise from a third-party.

Think of it as buying a used car... would you take it to a mechanic to get it checked out, or would you trust the dealer if they told you it just needs a new set of tires and it should be fine...

I'd go for the inspection!