My guess would be a leaking port or other deck fitting, not the hull deck joint. Water can follow some tricky paths between the hull and liner, so where you see the water is often not related to where the leak actually is. Try to imagine all the paths that water could reasonably take by gravity from higher up in the boat to where you see it.
The 376 has quite a few access panels and interior lights that can be removed. It may be worthwhile to look in those to see if it's damp. A good light and a little mirror can help. I've found & repaired three different leaks that way.
The best way to be sure where it's leaking is to see the water near where it's entering (obviously). But sometimes that's not possible. Then it has to found by a process of elimination. Usually that can be done when it's raining, or by having someone watch the area inside, while another trains a hose on the suspected leak. Be methodical and patient with the hose. It may take a while for the water to become visible inside.
Due to the way it's built, it seems the hull /deck joint could be ruled out or proven quite easily on the 376 by spraying just the rub rail area with a hose.
I've read that the "pros" use a shop vac or other blower to pressurize the boat, then apply soapy water to the ports, etc. Supposedly, any leaks will show up as bubbles. There are quite a few air leaks in any boat, so I'm not sure how effective that would be unless a lot of sealing is done first.
It can be quite frustrating to take something apart, reseal it, then find it didn't fix the leak. Much better to be sure where it's coming from first, even if it takes a while to find it. In any case, I'd like to hear if it actually was leaking under the rub rail. Good luck.
