I'm also looking for a replacement. The Chinese rubber was cracked when new, and now it's really looking shabby. Re-rigging the boat just to replace the boot is out of the question.
What I was thinking was to get a replacement boot for an automotive front wheel drive constant velocity joint (CV boot).
They're available as "split boots" that can be installed around the cables and hoses, and then glued shut along the seam.
The problem is, the dealers and web sites only list applications for specific vehicles, and don't list the sizes.
Anybody work at an auto parts house that can measure the boots?