In principle, this should be little different from voluntarily getting off a ship before the end of a cruise, which I have assisted others to do from time to time. The key is communication - make sure the ship knows what you're doing and that they're OK with it. If you need to make travel arrangements in advance to get from one port to the other, then you probably need to liaise with the cruise line before the cruise. You don't want to just disappear, otherwise you'll end up inconveniencing the ship when they think you've gone missing and missed a departure.

The only problem that I can think of off the top of my head depends on where you're thinking of doing this. If your trip might put the ship in violation of an anti-cabotage law (like the laws relating to coastwise traffic in the US), then they might object because they could be fined for the violation - as happens when people get off non-US flagged ships having only travelled from one US port to another without calling at a non-US port.