Do you have teak cappings on the toerails?

I’ve had a couple of “mystery leaks” and found both were in the hull/deck joint under the teak toerail, where there were voids in the sealant.

Unfortunately it means lifting the toerail (a right pain) and filling the joint with Sika, but it was the only answer and solved the problem.

Try pointing a hosepipe at the toerail and see if you get water in the boat. If you do, then that’s where it will be.

Best of luck