> There is nowhere close to enough biogas for ocean going vessel.

The total amount of biogas production is around 2.5 exajoules (2.5*10^18) per year. The amount of diesel used for intercontinental shipping is 213 million tons, or 10^19J (given the 45MJ/kg energy density).

So we need about 4x the current biogas production. That's entirely doable, especially with the power-to-gas transition.