A 2 x 12 will tolerate a fairly large distance between supporting members below it. I dont think id go more than 24 inches. You could use a 2 x 4 on edge. Its 1/2 inch too tall, but that should provide a little "wiggle room". Drill holes at an angle and secure to the existing deck with screws. Id use stainless steel or at least coated deck screws. You could look at the man made decking for the face inside the pool. It may need a new edge, but it should work like wood. Leave the new deck boards a little long over the coping temporarily installed. Take the face boards and clamp it in place so that it makes the arch you want. Mark the deck boards and cut them to the mark. There is also PVC trim boards you can get at Lowes or HD. You could use the man made decking for the whole thing too, but Id make the supports have a 16 inch spacing.