Here is a small mod that adds water slabs to StarMade. This can be used on both singleplayer and multiplayer.
For this to be used on multiplayer, you must follow all steps that you would for it to be used on single player, and then upload the StarMade files onto the server, using said files for the server. Any player can then join the server and use the modded blocks without having to install the mod, as it is completely serverside. If you choose to use these blocks on your server, please credit me for their creation.
Here is a short video that shows the installation process: