A diorama featuring, at the bare minimum, Baby Jesus, The Virgin Mary, and Joseph; plus, if the means allow for it, shepherds, magi, angels, and/or various livestock, often inside a manger, which has been completely submerged in water.

There are three basic variants:

  1. The most basic, the Snowglobe, is a small glass (or more commonly these days, plastic) dome with a nativity scene affixed to its base, filled with water and a white confetti-like substance which, when the entire unit is shaken up, disperses about and gradually resettles, creating the illusion of snowfall. These are easily the most common type of underwater nativity set, and can probably be purchased at your local retailer.
  2. The Aquarium Nativity Set consists of a small wooden manger and statuettes for each of the characters, which is then placed inside a fish tank. Some detractors are quick to point out that bettas and guppis were never observed by the gospelists to have circled about the skies at Our Blessed Saviour's birth, but strangely enough, these purists don't even bat an eye when they see Jesus under water. On a larger scale, proving that for Italians, nothing is too sacred, some small Italian villas have been known to capitalize on the season by carving nativity sets into large underwater rocks, which can then be viewed from the shore, the movement of the waves giving the illusion that they are alive with movement (okay, so actually, they carve the rocks on land, and then have them submerged into the local lake).
  3. The "Living" Underwater Nativity Set: in this, by far the rarest and most elaborate version, The Holy Family et al. are played by actual actors wearing SCUBA gear, inside a large tank or natural body of water. For reasons both practical (you ever tried getting SCUBA gear on an ass?) and legal (child endangerment laws generally prevent keeping an infant child underwater for lengthy periods), living underwater nativities are few and far between.