"Thanks, but no thanks." you say to the man. "I'd prefer to become a frogman instead." You walk off.
The old man grinned. Then he mumbled something. Then, before you could ask him what he was doing, the changes happened, making your clothes [apart from your undies] come off. Your skin turned green, your eyes swelled up, and you felt your feet getting bigger, and your toes lost their nails and became webbed.
You tried to scream for help, but all that came out was a garbled croak.
you were now a [literal] frogman!