Accepting that there must be a balance between the interactive and the immersive concepts of the game you present, where is the point of difference where the players are concerned?
As you sit down to play as a DM, you have in your mind a series of possible incidents that are likely to occur, based upon the position of the players and their expressed desires. Last running, you left the players having just reached a particular town where they have stated their intention to hire a boat. Then the session ended and you have a week or two to guess at what the party will actually do when the next running starts. Possibly, the party will change their mind and not hire a boat - but honestly, this is not likely. Still, you have to be prepared for both.
IF they change their minds, and decide to suddenly travel overland to their destination, or pick a new destination, you might simply throw an obstacle in their way for a session to give you time to prepare. But IF they do decide to get a boat, it wouldn't hurt to make up your mind about who the captain might be, how the boat's crew might act, what the crew and captain might intend to do once they're at sea and so on.
This is not railroading. The party is going to get some kind of crew. Unless they sit down and interview every crew in the harbor - which will be time consuming and which might challenge you to come up with creative crew ideas on the fly - they're going to get the crew you give them. What is important to recognize is that they COULD interview all the ships. They could decide NOT to get on with the crew they've just been introduced to. They could sit and wait until an entirely different ship comes into the harbor. They have options.
Something is a railroad when, no matter what the party decides to do, they wind up with the crew the DM wants. If they take ship B instead of ship A, its the same crew. If they wait for a ship, its the same crew. If they interview every ship in the harbor, and no matter what they hear or see or are told, it turns out to be the SAME crew the DM wanted from the beginning. There are no options. And this is true even if the DM does not let on that there are no options.
What it means is that railroading is something that happens in the DM's head long before it gets to the gaming table. It is a PHILOSOPHY of dungeon mastering, based fundamentally on laziness. It says that as the DM, "I am entitled to make use of whatever adventure I have conceived, for the good of the players and for my own convenience." Many DMs will defend it ethically on that specific basis. They will argue, "A good, interesting story is GOOD for the players ... and the more work I have done on that story, the better it will be, because it will be a deeper, more meaningful experience."
It is a very dangerous mindset. It is substantially a belief the DM holds that in any situation of the game, what the DM says is good must be good because the DM has worked on it (conceived of it, what have you). The DM further will tend to believe that because he or she that is running the game has more experience, this further proves the value of their judgment in these matters. "Trust me, I'm the DM," is a terrible sentiment and an awful, self-righteous way to determine what will interest a given player.
It is a common argument in other fields, and is refuted directly by the axiom that Home Rule is better than 'good' government. The smug satisfaction that a DM makes a game great simply because he or she is the DM, and knows best, is a patriarchal pile of bullshit that many players unwilling to walk out on a game will have to live with.
See, it isn't just an argument that railroading is good or bad ... it's the argument some hold that railroading - to some degree, they will always qualify - is necessary. They believe that it's necessary to produce a 'good' running, necessary to make the best use of the DM's time and so on ... and they will not be dissuaded from the belief because, in their eye, the proof is in the countless sessions they've run where the party didn't know they were being railroaded. If the party doesn't know, its all right, isn't it?
There is a great deal I could say about the ethical quality of this argument; your government has been making the argument for, oh, about 6,000 years, and so has your spouse, your children, your employer and a host of people who blithely sell you flour mixed with rat turds and lettuce with possible - but not proven - e-coli. What you don't know won't hurt you ... at least, of course, until it does.
It isn't just more ethical to quit screwing over your party in this particular way, it is one fuck of a lot harder, too. If you don't pre-plan the shipboard denizens of your ships, you're bound to slip into the laziness of always putting on a boat the first idea you have; and if you do pre-plan, you're bound to stick stubbornly to whatever it is you've designed. Neither gives the player much of an option when the time comes to play, and thus the only tactic you have is to LIE like a rug and say no, you never did have anything set in stone when the party started up again to look for a ship.
Which is utter bullshit, because of course you had something set in stone. You had the idea for a boat waiting for them. You'd be an idiot not to. One would hope you've spent the intervening week or so thinking about the party and the adventure, and that you have the wherewithal to be inventive and conceive of some kind of captain for the ship and the crew too.
What you have to teach yourself to do, however, is to A) give the party an honest chance to make up their mind about real clues about how the captain might act once at sea ... and B) to be prepared as a DM to throw away all that you've created and start from scratch.
And yes, that is so much harder than just railroading the party, you're probably not going to do it. You're probably going to justify your deception to yourself in the ways already described, or in exciting new ways of duplicity, and go on cheating your party out of their choice for their own good for as long as you play.
If you haven't understood the error of your ways yet, let me try this: if you are being this wonderful DM to your players, giving them the best games you know how by making sure every door they open is the one you want, then you haven't very much respect for your players. You have even less respect for yourself, knowingly deceiving these people regularly that you claim to be friends with ... and the more successful you are, the more you have to wonder. They're obviously not your friends in the way that they know who and what you are; or they're such loathsome little worms that they don't care how they're disrespected. If the former, how many would be your friends if they knew; and if the latter, how many of them would you want as your friends if they were just that weak-willed?
Meh. It's only ethics.