The swift ocean currents can make travel difficult, so proceed carefully. Surf to the first spot of shallow water, then grab TM06 (Toxic) from the next. Ride the northernmost of the two westbound currents to fight Swimmer Wright and pick up a DeepSeaTooth. Take the southern current west to reach the large island on Route 18.
Climb the staircase to the top of the cliff and battle Hiker Jeremiah. Trek through the tall grass to reach the only building on the island; a girl inside will restore your Pokémon's health. Talk to the Pokémon Ranger here to receive an Egg he found in the Relic Castle. The man in black is looking for a Rare Bone; wild Crustle sometimes hold them, which you can sell to him for 10,000.
If you have a Pokémon that knows Strength, you can cross the bridge to the south and drop the boulder to find a Max Elixir. Go down the staircase and shove the second boulder east to fight Backpacker Sam, and pick up a Dragon Scale. Head to the northwest, then double-back through the tall grass on the island's west side to find HM05 (Waterfall)! Go north and climb another staircase to fight Veteran Ray, then continue east across a second bridge and climb down the last staircase.
Ride the southernmost of the two eastbound currents to wind up in front of a small island. Grab TM24 (Thunderbolt) to the south, then check out the P2 Laboratory. The building is devoid of people, but is cluttered with machines. Surf southwest between the currents and around two Swimmers to reach the large island's interior. A Heart Scale lies on a secluded part of the beach to the south, while Battle Girl Hillary and TM19 (Telekinesis) are farther west.
Source: Bulbapedia/ Appendix:Black and White walkthrough/Section 8