I'd first like to know where the source of the leak is. Unless a hole has rusted though the tub, it should be able to be fixed. Did you notice any rust prior to the purchase of the house?
I believe home inspectors are limited in the scope of their inspection and cannot take things apart for examination. Home inspectors are a great source for determining the condition and serviceability of a structure, but there is limitations to their findings and you only get their opinion of the conditions that exist at a specific point in time, they do not warranty that any part of the building will last or preform in any manner.
Also, who hired the home inspector, you, the listing agent, your real estate agent, the seller. Point is, next time make sure you hire and pay them, real estate agents don't like to find to many problems less the house won't sell.