When im trying to place it on the water, it says "cargo ship must be placed on rails", and if i try putting in on rails on the land it says "ship needs to be placed on straight water ways with both sufficient space to both sides".

any idea what is the problem or what am i doing wrong?

Rails means in this case the blue waterline
It behaves like normal locomotives just on water.

You cant place the ships on the diagonal waterways only the horizontal or vertical ones.

I'm trying Carco Ships for the first time and I'm also having trouble placing ships on the rails.

I succeeded when I first tried, then I went to do it again and it wouldn't work. (a very long straight length of water rail)
So I copied and pasted my water route from the place where it worked to the place where it didn't work, and that fixed the problem.
So I'm not sure what went wrong... (it works now)
Perhaps a gap in the rail?

It's weird, but sometimes waterways get "corrupted". The solution is to mine them and place them down again.

In the F4 debug options, turn on "show-debug-info-in-tooltips". Hover your mouse over the offending water way and see if it is called "straight-water-way" or "straight-water-way-placed". If it is called just "straight-water-way", then it needs to be mined and rebuilt.

There are two kinds of rails, "straight-water-way" and "straight-water-way-placed". The mod is supposed to detect when you laying down "straight-water-way" and replaces it with "straight-water-way-placed". If for some reason it doesn't get replaced, you can't put ships on it (even though they can drive over it). This usually happens when a different mod tries to copy and paste waterways and doesn't raise the normal player- and robot-building events so cargo ships can replace the rails. If you get a blueprint with the wrong kind of rails in it, that can cause problems too.

What is your list of mods that you have this problem with?