How do you get into lake valor after explosion?

After the explosion at Lake Valor in Pokémon games like "Pokémon Diamond," "Pearl," and "Platinum," you can access the lake by using the Surf ability to navigate across the water. First, you need to defeat Team Galactic at the nearby headquarters in Veilstone City. Once you've done that, return to Lake Valor, and you’ll find the waterway clear for you to explore the lake and its surroundings.