Why is the sea like glass?

Hi, been noticing this a lot lately, but the sea can be completely flat. I’m not talking about inland bodies of water but coasts and the sea.

Seems to be especially prevalent in Asia. I was flying about Singapore yesterday in a small aircraft and the water was so mirror like it felt like an optical illusion.

Even if the sea is calm it doesn’t look like a polished mirror does it? You would have smaller waves. And as I said I’m not in a commercial airliner here at 25-30000 ft here.

Anyone experience this?