It's not well advertised, but you can book voyages on many cargo ships. They just give you one of the crew cabins and pack some more food. Expect to be the only passenger.
There are some issues though. It's slow (slower than an ocean liner since ships are more efficient at low speeds). And it's a cargo vessel, so the cargo sets the schedule. If there's an issue with the cargo that delays the ship by three weeks, you journey is delayed by three weeks. There also just isn't much happening. You have a room, a mess hall, a crew of maybe half a dozen to a dozen people to talk to, a ship to walk around on, and not much else.
It's more of a "the journey is the destination" thing. Accordingly there are a couple youtube channels documenting such journeys
As an astronomy buff, I would do that just to observe the stars from the ocean.
Do you know if they can turn off the bright lights at night? Even for just an hour, coordinated with the crew or captain? I might consider such a journey. I've never seen the southern hemisphere sky.
I think there are no climate controls to cool/heat the air to acceptable human levels inside the parking area. I remember seeing on the local news that the inside of the ship looked like a tightly packed parking lot, which seemed to be done by machinery to maximize occupancy.
