Hello Ennobler and welcome to the forum
1) Ships always have a range of travel. I believe the scout unit has a range which seems large but he always comes back for food. Ships could have a fisherman supply food but beer is more difficult.
In the current implementation, an expedition ship can sail unlimited distances without consuming any wares. Since it can also be used as a scout, there´s no point having a scouting mode with range limits or food consumption – it would not be used, players would use expeditions as scouts instead.
2) Ships come in different sizes. I am guessing ferries would be built at the shipyard. You could have a large ship, a regular ship, and the ferry. I don't know how the game decides what to build when one building can produce multiple units.
3) Ferries are a nice idea. They should have a range too. Is there a limit to the distance a carrier can go?
They are rather more than an idea already
Ferries are built at a ferry yard. They work completely different from ships: They serve one predefined path called a waterway, and can carry only wares (only 1 at a time). Ferries are a kind of carrier that moves on water.
See https://code.launchpad.net/~widelands-dev/widelands/ferry/+merge/351880 and the associated bug report
I don´t see the point of having several ship types…
3) If ships are going to be able to attack, will multiple ships attack like soldiers?
Sea attack is a feature that has been discussed many times and is wanted by pretty much everybody, but no definite concept has been decided on yet as far is I know…
4) If we are getting into rivers, they usually have currents. Where I live, small vehicle ferries are secured by cables or they would be swept down river. Too much? Maybe.
Ferry accidents were discussed and quickly rejected because it has too much undesirable randomness. Same for ships IMHO.
5) Could a ship carry a scout and wait to pick him up?
We have ferries in the development. Should they be able to scout, too?
IMO, they would be too cheap and too powerful as scouts. If ships ever can do scouting, this question will arise
-1, because ferries deliberately are less than just small ships