Is your actual engine up to temp? if so then I wouldn`t think it is your thermostat.Is water circulating ok in the reservoir when you rev the motor,if so then I don`t think it will be the water pump,other things to check are heater control valves which on some motors are electrically operated and are prone to seizing,if both heater pipes are hot then I would try removing them at the bulkhead,apply clamps to the hoses to prevent water loss and try flushing out your heater rad with a hose, just some things to check out.Good luck.
cdc, November 2013
