The problem is that it’s not possible to place land units(like trees) on or near water.
Same opposite comes for water units(like boats) which cannot be placed on or near land.
To make the scenario works as cool as possible, those placing limitations should be removed.
If anybody knows if there’s a way to do it I would be grateful!
There’s control R and control S which allows off grid placement and overlap placement. I forgot if that allows anywhere placement wrt land units on water, etc.
Yes, I know the off grid and overlap placement keys. But still can’t place most land units on water and vice versa.
And its would be Nice if we could place units inside a boat in the editor
Ctrl + R : Allow units to overlap/no collision.
Ctrl + S : Allow units on any terrain type.
Ctrl + G : Allow units to be placed off grid/pixel perfect placement.
Ctrl + A : Show unit squares.
It seems I can place any unit anywhere. Maybe your problem is fixed now? Which units did you have trouble with?
Thanks to you, it now works the way I wanted. Took quite long for me to figure it out lol.
Qaex: I think you mean transports, yes that would be a nice add.
What if you use the controls to place the units overlapped off grid any terrain literally on the transport, then you simply select them and right click the transport to make them get in?
I just tried the transport idea and it works. It would also let people know that there are actually units inside as nothing can move until you put the units inside the boat with right click 
Wow, that’s a good trick! May come in handy in scenarios that begin from the water for example…