Jack's eyebrows go up when Ianto rejects the more modest shorts, then he grins. When Ianto decides to do something, he certainly doesn't do it by halves. (Another thing that all three of them - him, Ianto and Spike - all have in common.) What else is he to do but pull Ianto in for a kiss to demonstrate how much he loves him?
Then he nods and lets his hand slide down to catch Ianto's and tug him towards the handy changing rooms nearby. "Yes, let's. I can't wait to get in the water!"