Is it methane?
Yes and no. It's part of natural gas... Lighter than air. See swamp gas. At sea it dissipates into the atmosphere. It doesn't stay in the water. I'm no chemist.. someone else should explain it.

methane | Definition, Properties, Uses, & Facts
methane, colourless, odourless gas that occurs abundantly in nature and as a product of certain human activities. Methane is the simplest member of the paraffin series of hydrocarbons and is among the most potent of the greenhouse gases. Its chemical formula is CH4. Methane is lighter...
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