Like Mario_4_Life said, Spongebob isn't in the game!
As for Sirena Beach - Episode 6: Scrubbing Sirena Beach...
The best what to clean it all up is to lightly hold R while running - that way you can cover more ground quickly and use less water.
Start with the beach and work your way to the upper levels - you can try throwing the little barrels - when they hit the ground - they will spread quite a bit of water around.
You could also try hovering to spray water.
Once you've cleaned most of it up - the timer will stop and you'll hear a chime - then you'll know you're done - go talk to the man on the beach where you started and he'll give you your well deserved Shine!
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