When recording the part where the Dutchman eats them (from an alternate ending) the cast is told by Stephen Hillenburg to make the walla "lighter" and less like the characters are upset.
The voice actors then start riffing as if they were being brutally digested.
Fun Fact: Did you know that Bill Fagerbakke improved "leedle leedle lee" during the recording session of Shanghaied? The script called for a long scream.
Here is an excerpt of the session, the session in whole is available on YouTube!