What sea creature is referred to as the invisible creature?

Answer 1

I think when we read about the invisible creatures of the sea what is being reffered to is Water Elementals, also known as Udines

Here is an exerpt from a website on them

Elementals are "Nature's Angels"

These nature spirits are governed by the Archangels

Like Angels, Elementals do not have a definite form or appearance. It's easy to determine if a picture is that of an Angel or a winged elemental. The winged Elementals have wings that look like a butterfly's and Angels have feathered bird-like wings.

If they choose to show themselves to you, they may choose a form that you will recognize and be comfortable with or will associate with the element they represent. A fire elemental may appear as a spark, a face in a candle flame or bonfire, or simply a sudden warm spot. A water elemental may be a cold spot. An air elemental could be a sudden breeze or tiny whirlwind in the dust. A smell, taste, or an unexplained pebble in your shoe could be evidence of the earth elementals at play