[Gray Lensman]

As a Steam user, multiple WORKING installation copies is really not possible, BUT, there is a work around this somewhat. Steam only allows you to run the game from the original installation location. (That's part of the Steam DRM), SO...

1.) Do a vanilla Steam installation.
2.) Copy (not Move) that vanilla Steam Dangerous Waters Folder to a different saved location. (It won't be able to run from that location).
3.) Add whatever mods you might want to the current Steam Dangerous Waters installation.
4.) Save that modded Steam Folder again in a different location. (Again, it won't be able to run from that location).
5.) If you have a third/fourth different mod installation, delete the current modded Steam Dangerous Waters folder and then copy your vanilla copy that you saved in Step 2 back to the Steam folder location and modify it to whatever you want, etc. Save that modded Steam Dangerous Waters folder elsewhere once again. (and Again, it won't be able to run from that other location.)
6.) Done correctly you can have multiple copies of different versions stored elsewhere (be sure to save the vanilla copy first, so that you don't have to download it over and over again to make different modifications.

In summary, whatever particular installation (vanilla or modded you've save elsewhere) just has to replace the current Steam Dangerous Waters folder.

Regards (Hope this Helps)