Taping The Stems - Vinyl electrical tape is used to waterproof the zipper at the stems. Clear duct tape is used to seal the remainder of the zipper if neeeded. A Velcro flap over the zipper is a better, though more complex, option.

Note - I no longer recommend the use of tape to cover the zipper as it leaves a stick residue over time. A flap is now recommended, though tape under the flap could be used if paddling in very rough water conditions, or when rolling.