MIAMI - The heavy rain which fell over south Florida on Saturday morning was expected to play a large part in determining the outcome later that afternoon in the $150,000 Calder Derby. And sure enough, it did. Good Morning Diva and Big Screen, neither of whom had ever raced on grass, proved the major beneficiaries when the Calder Derby was switched from the turf to a sloppy main track.  The two promising 3-year-olds separated themselves from the remainder of the field at the top of the stretch, and  then battled it out nose to nose before Good Morning Diva prevailed by a neck. It was another 7 1/4 lengths back to Musical Flair. Good Morning Diva, a son of Lion Heart trained by David Braddy, had made all three previous starts over the main track at Gulfstream Park. He won his maiden over a wet track before finishing a wide-running fifth in the Grade 3 Swale. Good Morning Diva broke alertly to show in front early before being eased back off the pace of Late Night Action by jockey Fernando Jara.  Good Morning Diva gained a short advantage leaving the final bend, was briefly headed by Big Screen at the top of the stretch before re-asserting himself to post the hard-fought decision. Csaba, who would have likely been the favorite had the Calder Derby remained on the turf as scheduled, made a mild run to reach contention on the second turn but could not sustain the bid,  eventually fading to finish fifth in a field reduced from 12 to nine starters following the surface change. Good Morning Diva ($5.80) completed 1 1/8 miles in 1:46.96.