What happened in the pool scene
After the incident with the old lady who started describing him, Moriarty stopped forcing the hostages to call Sherlock, probably because he feared that Sherlock would recognise him as Jim from IT. Meanwhile, Sherlock believed the missile plans was what Moriarty actually was looking for, but he was wrong. The reason Jim never communicated with him for the fifth case is because this case was about him and Sherlock (and John) and apparently he predicted that Sherlock, utterly excited, would use anything as an excuse to meet the promising nemesis. But Moriarty didn’t care about the Bruce-Partington plans (neither did Mycroft, actually) and consequently managed to surprise and impress the detective by throwing the USB drive into the water.
Anyway, Sherlock was the one who asked for a meeting (just like in Reichenbach). It seems Moriarty can never refuse an invitation coming from Sherlock. After one last big joke, when he made Sherlock believe that John is the villain, he appeared, showing all his power and intelligence. But there was a problem in this scene, I believe. The pool-Moriarty was very different from the phone-calls-Moriarty.
Jim first appeared as a gay man interested in Sherlock. In his phone calls his words are undeniably romantic; he even said that they were made for each other. However, while he went on with all the gay / sexual innuendos / jokes in the pool scene, he claimed that it was just a game aiming to make Sherlock “back off”. He claimed he enjoyed “playing gay”. Then he threatened him that if Sherlock didn’t stop prying, he would destroy him emotionally and eventually kill him. But there is a problem with all these threats and I think Sherlock was able to trace it.