Heh, I just asked Yellow about the boat prank. She said as long as no one gets hurt and nothing is damaged, then Leena, and Snype if you still want him to go along with the plan, will not even get caught. It's like Gary wouldn't even care that his vessel was floating all over the island. XD I'm going to do it, because it would be awesome. :D Do you still want in?
You never know. Don't forget, they have Pokemon as well that would be capable of tracking them. Gary's Umbreon for example. I'm sure that he'll see something. Heck, Snype and Leena may have been seen doing their first prank for all we know :3
How would they go about doing that? XD I still like the boat idea. Unless you wanted to do something different, or not do it at all. XD But like I said, they wouldn't be able to trace Leena and Snype back to their trainers... I think.