The Tomoebi is an event when a lot of salt flake snow falls and a cold water current flows through, the reflection will make it look like there are three suns and can only been seen from the bottom of the sea.
During Hibernation Edit
In this case it is the moon that is reflected, instead of the sun. It is seen from the surface because the sea is frozen. The ice acts as a prism that gathers light from the salt snow to create an illusory moon.