HOT ITEM: So what kind of boat does sell nowadays? The floating equivalent of a golf cart. . . . The Duffy Electric Boat Co., started here in 1970, turns out 16- to 21-foot battery-powered boats for cruising calm waters at up to 5 1/2 knots. . . . In 1985, only three were sold per month, but now, in spite of the recession, the factory is turning out one a day at $12,500 to $30,000 each. . . . Newport Harbor is still the No. 1 market, but recently a Taiwan resort bought four as water taxis.