A good way to source leaks is on a sunny day use a water hose to narrow down locations. For example, hose down the cockpit first and see if you accumulate any water. Wait till everything is dry and then do the same in other sections of the boat. Splitting her in to four quadrants is what most people usually do. Once you know the general area where the water is gaining entry you can then focus on whether it is a port, a deck fitting etc etc.
I can't remember whether it was Roger or Bruce but one of them tells a great story about a leak on a new boat where someone had patched a penetration in a deck with some fairing compound and painted over it. you could not see the hole with your naked eye but the water was wicking through a hairline paint crack and driving them nuts. they sectioned off the boat and sprayed down each small area and waited for water, finally finding the leak through the process of elimination.
It helps to have a second person, one on the hose and a second where the water accumulates watching for the water intrusion and then tracing it back to the source.
