By GINIA BELLAFANTE

A diamond may last forever, but nothing says eternal devotion like 31 lbs. of fish and an ample supply of stiff drink. In a three-minute ceremony held in her parents' Tokyo home, Japan's high-powered future Princess MASAKO OWADA slipped into a traditional silk kimono to receive the two sea bream (a male and a female), six bottles of sake and five bolts of silk that served as a formal marriage pledge from Crown Prince Naruhito. Following the offering, the Prince visited ancient shrines to inform the gods of his engagement.