Another point that I believe is critical for finishing 15-1500 swims. This set is 22,500 meters.
If you have not done this much yardage(errr...meterage) in a single swim before, don't try to do all 15 the very first tie.
I would recommend starting off with a modest goal of 3-4x1500 swims. This will allow you to get use to the distance, the repeats, the mental/phyiscal fatigue that goes along with the repeats with little rest.
As your body adjust, add 1-2 repeats until you have achieved your goal of 15