For the Alligator Pirate, answer his three questions correctly then find him at the pier (you need the Stuff Animal item to be able to answer all of the questions)
For the Forest Knight, when he's small he'll be at the Gym. Lift enough weights to impress him and then compliment and grapple him in battle.
To get to the scene at the shark's place you have to talk to the bat and shark at the bulletin board at night with at least one of them interested in you. For that to happen you either need to flirt with the shark and then apologize to him or talk to the bat about something he's interested in and then ask him to do something else once he's at stage 3. After the scene with them together you can get scenes with them individually by repeating the things that got them interested