Updated the original post, because Hooky's premiere date was outdated.
As it turns out, the episode originally aired on February 23, 2001 as a standalone U-Pick option. This is backed up by newspaper listings from February 23 including the episode as a candidate, and a VHS recording my friend owns from the actual broadcast. He recently was able to extract the XDS data from the recording's closed captions, confirming the air date beyond a shadow of a doubt.
Mermaidman and Barnacleboy II's premiere date is still the same -- March 3, 2001.