My 99 iszuz rodeo V6. Is running hot it's only got water in it not antifreeze

Water bubbling in the reservoir is often caused by either an air pocket in the cooling system that is not resolved, or more likely an issue with an internal engine coolant leak, allowing exhaust gases to get into the cooling system. This would be a serious issue and requires testing and possible engine tear down to inspect. There could be other causes as to why your engine is getting hot, a restricted or inoperative thermostat, a cooling fan issue, a radiator blockage...the list goes on. But the end result is likely a large problem inside the engine.