Drowned Hopes — Donald E. Westlake

Arriving home at dawn after another failed burglary, John Dortmunder is horrified to find his apartment occupied by an old cellmate everyone had supposed (and hoped) had been jailed for life.

Tom Jimson needs Dortmunder's help. Thirty years ago, before his last prison stretch, Tom had pulled a big job up near Albany. A very big job. $700,000 was buried in a small, upstate valley town. And while Tom sat in jail, the state of New York turned the valley into a reservoir. The stash is now under three feet of dirt and fifty feet of water.

Being the nasty sort of fellow he is, Tom's plan is to blow up the dam, flood the countryside, and grab the cash.

With the fate of nine hundred small-town nobodies hanging in the balance, it falls on Dortmunder to formulate an alternate plan for retrieving the loot. Aided by Andy Kelp, Stan Murch, Tiny Butcher and eccentric computer genius Wally Knurr, Dortmunder takes the plunge. And fails.

This, of course, necessitates a second attempt ... and another. And as each successive plan fails, Tom's dynamite finger gets itchier ... and itchier.