Go along to Route 1, then Surf down the small set of stairs, and keep going until you get to an island. It's pretty easy to navigate, so go into the little house thing (the thing that you find at the end of each route). After you go outside, then you should Surf and follow the water until you get to the other island. Walk around to the other side of the island (past the house where you can heal your pokemon), then up the stairs, up the other stairs, across the bridge, then down the two stairs ( I think there's two). Then you Surf east on that side until you get the P2 Lab.