A few weeks back a beach in California had lots of this animal (if it is an animal?). Can anyone identify what it is?
They had a distinct blue bottom and a see-through top.
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As can be seen from the above image, this 40-80 mm long [source] organism has a translucent sail with a lower bluish body that includes (not visible in your image or mine) small tentacles -- see below (as well as here):
For more detailed description, see Scripps Institution of Oceanography:
A clear, chitinous semicircular to triangular sail sticks up above the water with a blue float beneath made of concentric circles of gas-filled chambers. Small tentacles extend below the circular chambers. The sail is angled left or right.
Like the better-known man o' war (see below), this carnivorous hydrazoan (think hydra or other related cnidarians like jellyfish), floats on the water's surface, using wind to direct its small, rigid sail while dangling tentacles below armed with nematocysts catch and stun prey. From Wikipedia (my emphasis):
Having no means of locomotion other than its sail, V. velella is at the mercy of prevailing winds for moving around the seas, and are thereby also subject to mass-strandings on beaches throughout the world. For example, most years in the spring, there is a mass stranding that occurs along the West Coast of North America
Educational video of these sailors available here: YouTube
Note: my initial gut reaction was that it was a small, beat-up Portuguese man o' war (Physalia physalis), often referred to as the Pacific man o' war (or even a "blue bottle") in the Pacific Ocean. A colonial marine siphonophore (within class Hydrazoa) made of many smaller "zooids". Like Velella, it also floats on the water surface with its "pneumatophore" (a gas-filled bladder that forms a dorsal "sail" that can be pushed around by wind above the ocean) while tentacles armed with nematocysts dangle below to capture/stun prey.
However, the size and morphology (especially small size of the "sail", lack of obviously large bladder, and the clear concentric-circle morphology) of your specimen all pointed to another organism (and clearly toward Velella as stated above).