Find and obtain the Jewel of Water.
In order to obtain the jewel and clear the event, Tomba must have reached level ten in his blue experience bar. He gains blue experience from Needlegators, the crows in the Haunted Mansion before its curse is broken, the Butamushi in Wobbly Wharf, and the fish enemies in Trick Village.
Once Tomba reaches his tenth level, head back to the Jewel of Water and collect it. The event is cleared.