The merraenoloths, who travel the Styx, actually
do ferry people all over the place. So this is another case where knowing the lore helps. From the 2e Planescape boxed set:
View attachment 363912
So there you have it.
As DM, you control the ship's destination and length of the duration. The PCs want to go to another plane? OK, but they have to pass through three other layers of Hell first, each one worse than the rest. Then it goes through several other layers of other Lower Planes. Then,
maybe, it'll reach a somewhat less evilly-aligned port. Also, fiends generally don't eat or drink, and if they do, it's probably something really horrible, like baby heads boiled in urine, and you can't fish in the Styx, so hopefully the PCs brought along enough supplies to last them.
And if you
really wanted to, remember what the feature says: "In return for your free passage, you and your companions are expected to assist the crew during the voyage." Now what sort of assistance would a yugoloth ask from its passengers? Something very unsavory, I'd imagine.
Don't forget about that "no cargo" rule. This can include general equipment. Anything that might push the PCs into even the "lightly encumbered" category might be refused.
In 2e, I guess you were actually expected to roll a d100 and see if you rolled that 15% chance the marraenoloth would betray the party. If you're feeling generous, you can have the PCs make an Arcana or whatever check to see if they know that little bit of lore.
And finally, it's the Styx. One touch of its water and you're
feebleminded. Another Arcana roll should tell the PCs this, if they didn't already know.
You (as the marraenoloth) inform the PCs of the rules. "Yes, I will grant you passage, but you must work for me; I cannot take your for free. When I collect the souls at each stop, you will be tasked with ripping off the arms and legs so they can be transformed into larvae. Disgusting, work; the souls do shriek so. I will be glad for your assistance. Don't worry; I shall show you the fastest, most painful ways to do this. It's like cutting the head off a chicken, which I have heard is something you mortals do all the time."
Then you (as the DM) ask: do you
really want to take the boat?
Of course, you would have these outs even if the merraenoloth's lore didn't say that they take passengers, or even if there had never been a merraenoloth monster statted up in the first place. The PCs are in
Hell. Any boat they find there is going to be Hellish by definition and almost certainly manned by fiends and damned souls.
Any trip is going to be awful. Everything on the boat is going to try to kill or corrupt the PCs.
(Oh, and if they try to kill the merraenoloth and take its boat, well, the 5e stats (IIRC) kind of imply that it and its boat are one--you can decide that killing the fiend causes the boat to be destroyed as well.)
