Jeff wrote:

Turning the key, nothing either.

....


If this is a problem, you can use a small relay (under $10 at Radio
Shack) so that only a tiny current has to make the trip through the
various switches towards the starter. The small relay switches the
solenoid (which is a larger relay) to run the starter.

One more thing that might "go without saying" but I'll say anyways: if
the boat has the "neutral lockout" switch, the boat must in in neutral
to start! If you can look in the shift lever mechanism and you see
wires there, it means there's a switch. Sometimes just jiggling the
shift is enough to start the engine.