Is it worth it to book your next cruise onboard during your current cruise?

I’d say yes, but look at the details. Sometimes you can get locked into a small, specific window, while other times you have a broad timeframe to choose from. I would default to yes, with some fine-print scrutiny. Your best savings can also come from seasonal promotions and/or last-minute pushes by companies to fill a ship. Last-minute pushes can be public, but they're usually held by travel companies or agents with enough numbers to help fill the ship. I hope that makes sense and helps.