I don't think it matters from that height. No matter how you hit the ground, you are dead. The death will be pretty much instant too.
That's stupid. OP should focus on getting as high as possible. Everything else is secondary. With the huge anxiety that will build up there while standing there on the 15th floor and jumping, it's completely unrealistic to expect, having enough of a clear mind, to think of whatever they trained in the nearby swimming lake/pool. It's all about the height.