Is it ok to use in salty water?
If you clean it well after its in salt water it shouldn't have any issues. It is rubberized and if you don't remove the salt it most likely will dry out.
Can i change the ball head on this one?
How is the ballhead attached to the tripod? Via a 3/8” mount?
There is a "foot" with a flat head that is used to screw into the camera. Pretty sure its a standard 3/8" thread. The foot is then clamped into the tripod proper. It's all very snug, the quality is high in my opinion. By Charles W. H