Can bugs near my water cause diarrhea. like gnats or mosquitos ?

Let me try to sort: this out. Public health officials are in charge of monitoring and treating water to make it suitable for drinking; strict guidelines for purity apply. If you see bugs only flying around your glass of water, nothing adverse should happen...though you should go someplace to drink it where this isn't happening. If you see a bug in your water/water glass, discard it and get fresh water and glass.