[I'm not sure how, but I found Sebastian's original pic on google, the first try! ]
Sebastian didn't have any idea where his room was so he let Katrina show him. He then searches the room for a bathing suit, gets dressed, then jogs to the elevator. Moments later the elevator opens, allowing him to go to the level where the pool was located. Thankfully the two mansions were very similar so he had no problem finding proper sub-basement.
Arriving at the pool he walks in and sees Katrina and flashes her a smile. He does a jog over to where she's seated and says, "Hey. This place looks almost exactly the same as I remember. There's a few things out of place like, MY ROOM! But otherwise very familiar." He glances around to make sure no one is with them, then sits down beside her and says quietly, "So are you now going to give up the prank? This is a prank isn't it?"