Pete, That depends on a lot of things,
1 cast iron tub or fiberglass?
2 where is the leak, next to the drain or on a radius (curve at the bottom)
Let's assume it is cast and it is next to the drain. get an epoxy filler like JB weld and follow the instruction on the label. BUT FIRST make sure the repair area is dry (wipe it down then use a blow dryer) after it sets sand the rough areas off till smooth, then get an appliance repair paint (appliance store) to match the color as close as possable, that should take care of it. If it is fiberglass, go to an auto parts store and get a small fiberglass repair kit follow the instructions and do the above steps with the gel coat on the fiberglass.
Brian
January 2009