As Gary suggested, hatch covers would be my next bet....
only takes a fine grain of sand or grit to break the 'seal' to a point where a decent volume of water can enter even tho you've tightened them up as per usual.
One old trick I learnt years ago was to coat the threads in car polish and then buff them off like one would a 1/4 panel, the polish free ups the threads to better than new and unlike silicon sprays etc it doesn't act like a glue and hold the grit in the thread


I'd still be inclined to do the soapy water trick again, only this time I'd do the areas Gary said to check...
hatch covers and hatch ring to hull, bridle tang points on hull, centreboard cases and trampoline tracks.

& seeing you've done a few miles of towing of late I would add to that list the actual beams, jiggling around on the trailer may have loosened them off a tad, enough to break the seal... well that's anyones guess, but armed with a bucket of soapy water you may aswell check it all over