I had the same problem last week. Couldn't figure out where it was coming from. Most likely, it's coming from the bolt holes through the transom. Mine was leaking through the port trim tab holes. Because I place wedges (made from plastic electrical wire ties; the 2" tips from the really small ones that measure 1/64" width) to move the trim tabs positive or negative, water found its way through. I got some quick gasket maker and applied a small bead around the holes, then put the hardware back on. I ran her today at 36.4 mph and not a drop, 3 runs. I didn't even have a dry run like that since I got her out of the box. I still leave the 2 small sponges on the bottom, on each side of the stuff tube, against the transom just in case.

Don't put too much grease on the flex shaft or it will find its way up to the coupler and spray all over the place. Just a real small bead along the length of the shaft, leaving the first 1" or so free (the end that goes into the coupler). Too much will also rob you performance wise.