Hi Chris, welcome to the Forum, and congratulations on realising that there is something abnormal about this engine and wanting to do something to correct it.
1) I would normally expect the engine jacket water outlets to be about 75C but some engines should be run hotter and you have to check the manuals to find out the correct operating temperatures and pressures.
2) Have you double checked what the actual outlet temperature of this engine is? Maybe the thermometer is faulty, if you can hold your hand on the jacket water outlet pipe it is too cold.
3) If the engine really is too cold, the most likely explanation is a problem with the thermostat. On two ships now, I have seen the wax bulb thermostatic elements of the auxiliary engines were removed by a previous Chief Engineer in response to a problem with the temperature being too high. " Everyone knows that removing the thermostat will lower the temperature" as one Chief wrote in his monthly report to the Office. In fact the cooling water coming out of the engine has two choices, go straight back to the pump suction or struggle through a cooler first, so removing the elements is most likely to raise the temperature, not improve cooling. (The thermostats expand and close the cooler bypass passage when they get hot)
4) Depending on the type of thermostat many different things could be wrong, if it is mechanical it may have simply seized up if the cooling was drained out for a long time. If it is electrically or mechanically operated, maybe someone just shut off the air or power while they were working on the engine and never put them back on.
5) If there is an adjustable set point for the desired temperature value, maybe someone has just set it to the wrong value.
6) Some thermostatic valves, particularly those types driven by an electric motor have an emergency mode, in which the electric motor is disengaged and the the valve moved by a manual control wheel. That leaves the motor turning but not moving the valve. I have also seen one of these assembled incorrectly, the valve spindle and motor gear box shaft were different diameters and clamped together by a split bush with different bores at each end. Some one had put the bush back upside down!!! The small bore was tight on the large shaft and the large bore wasn't even touching the small shaft.
I hope this information helps you, Let us know what you find, and we can give you some more advice.
It sounds like you are on the sort of the ship where you have to learn fast in order to survive, Good Luck.
It is always better to ask a stupid question than to do a stupid thing.