Under as a preposition can mean 1 - in a place which is directly below: he hid under the table; the coin rolled under the piano. 2 - less than: she is under thirty; he is under age; it was sold for under $100; he ran the mile in under four minutes. Below as an adverb means lower down: he stood on the hill and looked down into the valley below. As a preposition, it means lower than: the temperature never goes below 25?; if you look below the surface; you shouldn't have hit him below the belt; his marks were considerably below (the) average. Underneath as a preposition means under/beneath: she wore a long woollen cardigan underneath her jacket; he was sheltering underneath a chestnut tree. As an adverb it also means under: he was wearing a thin shirt with nothing underneath. Beneath as an adverb means underneath/below: he looked out of the plane at the mountains beneath. As a preposition it means under: have you looked beneath the cooker? He thinks it is beneath him, he thinks it is too insignificant/too unimportant for him to deal with.

If there's a difference I can't think of it. 'Under' and 'below' are more likely to be used because they're quicker to say, but that's about it. You could use any in the following: ____ the ocean... ____ a table... ____ a roof... ____ a cliff... etc.