What does an insurance company cover on a blown engine

Depending on the cause of the engine failure, they might cover the entire cost
of an engine.

For instance: An acquaintance of mine had some work done on his Volvo diesels.
He paid to have some zincs replaced, among other items. The technician
apparently screwed one zinc in a turn or two, intending to tighten it down with
a wrench or a socket a moment later. Problem was, the the technician didn't
tighten the zinc down and it vibrated out when the boat was underway. Engine
overheated to the point where it sustained internal damage, acquaintance got a
brand new engine for the price of a small deductible.

When my engine failed, (hydrolock) I didn't expect to get any help from the
insurance company, as per the language of the policy. Actually, the adjuster
suggested that Boat US might be willing to pay to have the engine pulled and
torn down to determine whether the engine failure was a covered event or not-
and I figured that would be fine by me since the engine had to come out for
rebuild or replacement anyway. In the end, Boat US was able to reach a
conclusion about the cause of the failure without pulling the engine.