Pokémon Diamond and Pearl/Acuity Lakefront
There are 2 Galactic Grunts standing in the way of the entrance to the Lake. You must beat the 7th Gym Leader in Snowpoint City before one of the Grunts moves so you can go to the Lake. When you enter the Lake, Commander Jupiter talks to you and then leaves, heading back to Galactic HQ. Your "friend" then talks to you and also heads out of the Lake to get stronger. Go back to Veilstone City to GHQ. There are no battles here. But there is an item if you surf to the upper right hand corner of the Lake and walk through the grass a little bit.