The Mega Bucket was a proposed construction by Plankton who wanted to build it on Goo Lagoon. It would have been like the regular Chum Bucket but much bigger, if it wasn't for SpongeBob not being assertive. It appeared in the episode "Walking Small."
Plankton was planning on building the Mega Bucket on Goo Lagoon. He couldn't do it alone though so he tricked SpongeBob to do it for him. Plankton probably put it on the beach to attract hungry customers on the beach. The Mega Bucket might just have been a bigger but worse version of the Chum Bucket. The food might have been the same, but they're may have been some new items on the menu.