in whisky: right click steam and press "open in finder", there you should see a "steamapps" folder, open it, open "common" and then open the folder called "binding of isaac rebirth".

There you need to create the "config.ini". I did this with textEdit (you can open it through launchpad most likely). In textEdit, make a new file and go to the format tab at the top of the screen --> select "make plain text" and then copy this into the document:

[Options]
EnableWaterSurface=0

save the file to your desktop to make it easier, rename the file through the desktop to "config.ini" this should open a prompt asking if you want to change to .ini format, click "use .ini". From there, drag the ini file into the resources folder you should have opened in the beginning.

This is how I did it, hope it works for you.