If the stitched seams of a boat cover are leaking, there is often a problem with the stitching.
Most yarns used by sail makers in the manufacture of boat covers, have a synthetic core and an outer layer of cotton. The cotton expands when wet, thus sealing the stitching holes. The hole is certainly always wider than the yarn, but this is not a problem.
Over the course of time, the cotton becomes damaged and no longer expands. Eventually, it is destroyed by rainwater or through cleaning, for example.
The result is that the stitching hole remains open, easily letting rainwater in. At the same time, this increases the risk of damage.
Thus, check regularly whether the stitching is still in good condition. Run your nails over the stitching vigorously. Does the yarn break? If so, have it repaired by a sail maker.
You should also spray all stitched seams regularly with a good impregnating agent. That way, you protect the yarn and seal the stitching holes.
Sprayhood & Tent Protector is a perfect impregnating agent for impregnating seams, stitching holes and stitching.