Mast boot sealer???
The material that you are talking about is called ''Spartite''. West Marine still carries it http://www.westmarine.com/webapp/wcs/stores/servlet/product/10001/-1/10001/72052/10001/265/11208/2
That said, Spartite is not a waterproof system. Spartite is only a mast partner wedge replacement system. I.E. When you have a keel stepped mast, it should be ''blocked'' at the deck to minimize differential movement. Historically this was done with cedar or other softwood wedges. Direct contact between aluminum and the softwood wedges was less than ideal and so high density urethane foam has become the preferred wedge material. This is a bit hard to shape and install and so Spartight created a poured in place material.
Properly installed Mast boots should not leak if the mast in properly blocked to minimize movement. That said, by their very nature keel stepped masts almost have to leak because water enters the mast at the sheeve boxes and has no where to go but out at the mast butt and below deck wire penetrations.