How many watt-hours are theoretically necessary to produce 2kg of methane from sea water and air?
Too much for making it of any practical interest

This question cannot be answered easily, there are too many factors. Bytheway it would not be practically convenient making such a process. With $\ce{CO2}$ as your only source of carbon, you would need an extremely energy dispendious air separation apparatus to get some $\ce{CO2}$. You can get then hydrogen from water electrolysis and use  Sabatier reaction (you need a Nickel catalyst) to get finally methane.