London firemen were called Saturday to rescue Andy Perquin, 16, of 915 Colborne St. and Paul Morrissey, 16, of 36 Elworthy Ave from a clump of trees the middle of the Thames river. The youths canoe hit a submerged log in the mid afternoon and flipped over. Fireman Earl Smith and Gary Hewitt finally inched their way along a rope they had fired to the boys, but only after another fireman, Cliff Fraser, had been tossed out of a canoe in his rescue attempt.