After 17 years, the mystery of GTA 4's missing ferries has been solved by one of the original developers: "They were more trouble than they were worth"

gamesradar.com:

In March 2007, Rockstar revealed the first trailer for Grand Theft Auto 4, which prominently featured a boat docking at a ferry terminal. When the actual game launched a year later, the terminal was still there, but the ferries themselves were nowhere to be found. Fans have wondered for years why they were cut from the final game, and now one of the open-world game's original developers has revealed the answer.

"It wasn't really because of performance," former Rockstar technical director Obbe Vermeij reveals on Twitter. "It's more that they were causing other issues with physics (cars sitting on a boat). As well as people queuing and getting on and off. They were more trouble than they were worth."

The Liberty Ferry Terminal first appeared in the "Things Will Be Different" teaser trailer that introduced the world to GTA 4, and it got a fair amount of notice as one of the few concrete things anybody had seen of the game. It's not as if the cut content caused any notable backlash at the time, but the mystery has caught a fair bit of attention over the years. 

The 2020 video below from Badger Goodger is a pretty good summary of what the community knew about the cut ferries at the time. The terminal looks a bit broken down, but even in the final game it still has pedestrians queuing all around and police standing guard here and there. As it turns out, there are also other dock areas throughout the game where NPCs spawn in the same way, and a few likely indoor terminals with partially modeled interiors that are inaccessible in-game. As the venerable Cutting Room Floor wiki notes, the game files also include an unused boat map icon, a very likely sign that these terminals were meant to play a larger part in-game.

And now with Vermeij's comments, the mystery is all solved. He's been putting to rest all kinds of long-standing GTA mysteries in recent years, explaining the "embarrassing" bug behind the infamous plane crashes in San Andreas, and revealing the nature of the