https://en.wikipedia.org/wiki/Luciano_Federico_L
That ferry is where I got some of the numbers from. That ferry is a catamaran As I got further along in the mod, I changed some things to be what I thought would better fit in the game.

I have zero graphic design skills, but at some point I may try to update the graphics for the mod.

As for the number of cars allowed- I wanted one vehicle to provide versatility in passenger capacity so people's new train menus would not be cluttered up with multiple similar instances of the ugly ferry.

I will do some more play testing this weekend and think about what should be done.

One of the faster ferry I have heard of is Boeing 929 Jetfoil, that one had a speed of up to 45knot, or about 83km/h, still slower than the one you have in this example.
If you have a specific ferry in mind when creating this mod, I highly recommend you to change the mod's look to match the particular ferry you have in mod, which is most likely also better looking aesthetically.
And I recommend you to only allow 1 car for the ferry, similar to those bus mods, to be more realistic, as you typically don't chain ferries together in operation.

Maybe a fast one could be a catamaran

You are right about that. When I first got this ferry idea, I gathered some numbers from one of the fastest ferries in the world.
Since passengers will only wait a max of 3 hours at a transfer stop, having a single, slow ferry vehicle for a longer ferry trip will not work very well with the game's current rule set.
I will consider lowering the speed on this one, or maybe making another ferry with a more common top speed.

Nearly no ferry will go that fast max. 40 kmh is normal