[If he thinks he can get Ianto to lose control this easily he is seriously underestimating how far he and Jack have taken their secretary kink in then past, how often he's answered the phone whil Jack is fucking him or how many times one of them has hidden under a desk, working the other, trying to make them crack in the middle of a conversation with a coworker. It's a game he loves and is good at, and is more than happy to play here. So for the moment there's no obvious sign as to what he's up to; an uptake in his breathing, maybe, a faint flush to his skin but those could be due to the heat. The only thing strange is being the only one (apparently) in the pool but he can come up with an excuse if someone asks. He's good at this.
It's a little harder to maintain that facade when Spike nips at him, causing him to gasp sharply, unexpected. It feels good, having that bite immediately soothed by the cool water and he can't help but shiver a bit in delight.]