Or maybe Moriarty test their loyality to each other. Two snipers were logical, one for Sherlock and one for Watson. Sherlock and Watson could only see the laser points from the other one, so Watson see two points and could assume that he is not eyed by the sniper and could run away easily (it looks like he sit at one exit of the swimming pool).
But maybe there are no snipers at all, because Sherlock aimed the bomb and was no more scared to be shoot. A sniper could easily shoot him before he shoot to the bomb and blow all. Either he found out that there are no snipers or he knew that Moriarty simply didnt want to shoot him.