Some of the best stuff I've seen is black 3M windshield adhesive/sealant,the stuff they use for installing new windshields.It stays flexible and wears like iron.

Climber2, you have it backwards! Use a mixture of soapy water on the outside of the boot. Roll the top of the boot and push air out. Shows you where the leaks are and it won't take weeks to get the boots dry again.

super glue and duck tape....fix anything

Aqua Seal.
Thats a semi-rigid sealant use commonly to repair both wetsuits, and drysuits in the scuba industry.
It takes about 24 hours to cure, and can be used to fill in up to 1/4" gaps.
If you need a quicker curing time, buy cotol accellerant at the same time when you shop in most scuba shops.
Look up scuba diving shops in the phone book, and you can get Aqua Seal from them. Most towns of reasonable size have a dive shop or 2.

Put a garbage bag on your leg before you put on your boot and let 'er leak.