IT used to be said of bankers, formerly the stodgiest of business breeds, that they operated according to the "3-5-3" principle: Borrow money from depositors at 3 percent, lend it out at 5 percent, and be on the golf course by 3 o'clock.That laid-back approach to banking is as obsolete as the aphorism's interest rates. And no banker in the years ahead will have to hustle more than L. William Seidman's successor as chairman of the Federal Deposit Insurance Corporation - in effect the nation's top bank regulator. Mr. Seidman, the head of the FDIC since 1985 and, more recently, simultaneously the chief of the organization supervising the thrift bailout, announced he will retire in October. Banking's new chairman, not yet named by the White House, will take over at a time of more turmoil in the lending community than in any period since the Depression. America's banks - like its savings and loans - are staggering from recent blows, especially the downturn in the real estate market. The nation's deposit-insurance fund, tapped heavily to rescue collapsing banks like the huge Bank of New England, is nearly depleted. The new head of the FDIC (it's expected that the thrift bailout will be given to a separate executive) will have to navigate the banking industry through the current high seas. In the longer term, he or she will also oversee the large changes in the industry being proposed in reform legislation before Congress. The elements of the reform package are still being hotly debated, but they almost certainly will include changes in the deposit-insurance program, authorization of interstate branch banking, and p ermission for banks to engage in other lines, like selling securities. The ultimate tasks of the new regulator will be to preserve Americans' confidence in the security of the banking system, while smoothing the way for innovations that will make US banks competitive in the new global financial environment. The respected Seidman leaves big shoes to fill. President Bush must carefully select a successor who combines Seidman's integrity, no-nonsense executive skill, and ability to work with Congress and the industry.