Is it possible to connect the cargo ships using the Automatic Coupling System?
I would like to have the big ships "guided" by the small ships within a port. In other words, I would like to use the small ship as a tugboat.

That's a cool idea, but not something we can support. ACS doesn't work when uncoupling two engines from each other like a boat engine and a ship engine. Each half would be given the same schedule and they end up colliding.

That's right, I hadn't really thought about it.

Would it be possible in the future to make a ship that is compatible with ACS? For example, one like this:
https://www.hvcc-hamburg.de/site/assets/files/1072/binnenschiff_am_ctb.jpg

The current code mandates that ships be one wagon and one engine, and wagon-engine pairs can't couple to each other to keep weird things from happening. It might theoretically be possible to change this (assuming the "tug" in your picture latches on from the back and not the side?), but would be a major rewrite. The "tug" would have to be two locomotives back-to-back so that it could reverse away from the barge, could easily end up "backwards" relative to the barge, and the station stops would be an annoying array of ports and combinators. And you'd have to somehow correct errors by "shunting" manually, without the ability to get out of one wagon and into another (because they are on water).

mh... off the top of my head, I can't say anything against that. But anyway, thanks for the constructive and quick answer.
Otherwise, have a nice day / night (depending on what applies to you)

You too! Thanks for the feedback. I don't mean to be a downer, just don't see clear path to making it work. I have spent plenty of time with ACS on land so I know it's fun. If you figure out a way to do it in a cleaner manner by all means share it here.