WHY IN THE WORLD DID PRESIDENT BUSH CHOOSE TO HOLD HIS December 1989 summit with Mikhail Gorbachev on board heaving ships off the wintry coast of Malta? After the world's most powerful duo endured the so-called seasick summit, the President tweaked his younger brother WILLIAM ("Bucky") BUSH for suggesting the stormy site. It turns out that Bucky, an international investment consultant, had business ties to firms that could have profited from the choice of a tiny nation trying to boost its tourism. He was, for example, a consultant for CIGA, an Italian hotel firm that was mulling construction of a new resort in Malta at the time. He was also involved in a French firm that was planning to sell restaurant equipment in Malta. Asked about the connections last week, William Bush referred the inquiry to the White House, which responded simply that Bucky "did not stand to benefit in any way from the holding of the summit in Malta."