MostlyNumbers' mods CanalBuilder and Hovercraft add content that nicely complements this mods' additions.

There's an optional dependency on Hovercraft, but no dependency on CanalBuilder. I guess you omitted that to avoid confusion because MostlyNumbers hadn't been updated for a very long time and you made a replacement mod. However, the original mod has been updated last week. Now I'm wondering what to do: Should I keep your replacement mod, or should I install the original one?

An official recommendation from your side would be nice, and a dependency seems to be the way to go.
If you changed anything in your version of the canal builder mod, or if you plan to do so, an optional dependency on your replacement mod would be good, otherwise you should add a dependency on the original -- or perhaps even on both canal builder mods. :-)

dependencies are not for recommending other matching mods, but for setting the load order of mods when used together. mods listed by this mod as an (optional) dependency are loaded before this one (as opposed to the default alphabetical loading order). this allows me, if some other mod would usually break this mod (or there is a incompatibility in general) to let the other mod be loaded first and then fix whatever caused the incompatibility during the loading of the cargo ships mod.

mods listed by this mod as an (optional) dependency are loaded before this one (as opposed to the default alphabetical loading order).

Yes, I know that dependencies (except conflicts) are used to make sure the other mod gets loaded first. However: If that is all you want of dependencies, why using an optional dependency at all? Wouldn't a hidden optional dependency provide all you need? In case you don't know, you can make a hidden optional dependency with "(?) modname …" (note the parentheses around the "?") which will work like the normal one, but not shown to users.

From this perspective, it seems to me that the normal optional dependencies can have an additional purpose: endorsing other mods, or indicating that your mod has implemented support for a specific mod. In fact, I'm considering to add an optional dependency on Cargo ships to a mod I'm working on because there is that one item that gets unlocked when automobilism is researched -- but I think it should also be unlocked if your boat is researched first.

But back to the original question: Should I keep your replacement mod, or should I install the original one? Do you intend to continue working on the Canal builder mod, or was it just meant as a temporary replacement until the original gets updated? :-)

the canal builder mod i uploaded was just a temporary replacement, i will look into removing it again. so go with the original :)

i will think about the dependency stuff. for me it still would feel weird to add even optional dependencys to mods where there actually is no tecnial reason to do so. also i am not sure what the behaviour is when two mods both have the other mod as an optional dependency. or if that would even be possible. so it feels for me that adding dependencies whcih are not tecnicaly necesary might reduce the ability of other mods to add those dependencies when they actually need them.