Honda overheating

I can hear water running when I accelerate in my car. My car also overheats and when it overheats it starts to idle low like its going to stall. I can't see any leaks when replenishing my coolant though. Can this be my water pump? I had the water pump and timing belt replaced last year. I replenished the coolant through the radiator. I also noticed the heater does not get very hot when I turn it on anymore. The water pump is still under warrantee but it is not leaking. Maybe I will start with the thermostat first since it is the cheapest and simplest. But would the thermostat cause all of that?