whats this switch for
 

i just bought a 1996 donzi zx25 and there is a switch that says water system and i have no idea what it does.can someone tell me

Probably your fresh water pump. Do you have a faucet or hand shower?

my guess based on limited info is it is for the fresh water system. Turn it on and it should activate the pump. you probably have a sink and a transom shower that it goes to.

no sink or shower where is the fresh water tank at

It would be on the port side of the engine. If it is still there you would not be able to miss it. There would also be a nozel and hose that would be able to be pulled out by the swim step. Someone may have taken the tank out.