The trainer to the right of the hiker will be looking downward / south. If you can get him to walk out towards you, you will be able to grab TM19 – Seismic Toss behind him. If he doesn’t walk forward enough, you’ll have to come back after obtaining the third badge and use cut to chop down the bush on the right.
Enter the seaside cottage to find a man named Bill. He has been transformed into a Pokemon! Speak with him until he goes into the machine at the other side of the room. Interact with the computer to return him to his human form. Speak with him once more to obtain the S.S. Anne ticket. You’ll need this when you reach the next town.
Now that this is all out of the way, return to Cerulean City and take on the Pokemon Gym. If you’ve already earned the Cascade Badge, go south to Route 5.