On the line of horror character portraits, the first one was a greenish monster and the second one was a purple guy with red hair, but when the monsters had Shaggy and Scooby, they switched places.
Only two culprits are ever identified, but multiple monsters were active at the same time.
Shaggy and Scooby state that they can't swim, but they have often been depicted as very good swimmers in this and other series.