There are over 100,000 large merchant vessels. The rate for maritime customers is
between $250-$5000/mo. If you can charge them $1,000/mo that would be over a billion a year. Cruise ships will be on the upper end of that price range, if not using a higher bandwidth bespoke plan
Airlines will have similar needs and pricing. Their current internet service is bad and expensive so they will definitely switch. Easily a few hundred million in potential demand
Obviously not every ship and airplane will sign up but that still forms a substantial revenue baseline that isn't going anywhere and no similar competition is in sight
You also have other circumstances like Antarctica, Ukraine, and in-orbit communications where fiber isn't practical either.
Not to mention defense agencies. They already have a massive deal with the DoD both for civilian Starlink and building out Starshield