OK, that's a good start - you've isolated the source to "outside the hull"!
Now it gets tougher. The good news is, water always flows downward. So all you have to do is trace it back uphill! Then plug the leak, and you're done. Simple when you apply the proper methodology.
Sources of salt water intrusion include the shaft log (where the prop shaft exits the boat), the rudder shaft, through-hulls and related plumbing, and the raw-water side of the engine cooling system. A common source for a leak is the seal for the raw water pump.
Do you happen to have a salt-water head?