Lots of variations on the theme re this question.
For what its worth, I've had good success with White color duct tape "pro grade" (or some designation like that) marketed under the ACE brand at ACE hardware stores. Last winter I had several suspected source-of-leak locations in the bow area (such as all around the anchor roller hardware, the bow cleats, and the sampson post) that I knew required quite a bit of time/effort for hardware removal, repair and re-bedding properly with marine grade caulk. So as an interim solution, I covered all the suspected joints and bolt locations with the above mentioned white duct tape. The adhesive is water-proof and there were no leaks for the rest of the winter rain season. And I was surprised that six months later, when I started to make the permanent repairs, the tape came off OK. The tape's adhesive was still flexible. And the adhesive that did remain on the fiberglass and fittings came off easily by wiping with a rag moistened with acetone and/or lacquer thinner. Lacquer thinner doesn't seem to damage the fiberglass gel-coat that I have noticed. And the white tape actually looks fine on the boat.
The white duct tape can also serve as a temporary sail repair material I would think.
