For 30 bucks it doesn't make sense NOT to have one. Here is a guide from
Marine Engine Analysis
If you have not used a laser temperature gauge before you’ll be amazed at how much information this simple tool will show us about our engine. (See Tradeaboat April for full use instructions)
Begin by scanning the gearbox looking for hot spots. Pay careful attention to the area of the case around the clutch packs, and the gearbox output bearing. If any part of the gearbox is going to build up heat it will be during this bollard pull test. The complete gearbox should be under 80C, and 60C is typical.
Continue checking with the laser gauge along the cylinder head. It should show about 75-85C everywhere. There should be no hot spots. A hot spot would be more than a five degree C change in the head from one end to the other.
Note-Testing the area directly around the exhaust manifold does not count as a “hot spot”, as this will be hot due to the high temperature of the exhaust.
Check the oil temperature by shooting the oil filter or oil pan. The oil temp should be about 7-10C above the head temperature and not above 95C.
Check the inlet to the salt-water pump and outlet of the final heat exchanger. With most engines the temperature difference should be less than 10C. A higher temperature difference can indicate an engine producing too much heat (IE failing head gasket) or too little water flow (IE clogged salt water system).
Check the fresh water temperature inlet and outlet of the heat exchanger. The difference should be about 10C and stable. You should be able to move the laser along the body of the heat exchanger and show the cooling effect of the heat exchanger. By this method you can see how much reserve cooling you have left in the heat exchanger.
The pre turbo exhaust temperature should be about 300-450C. After turbo it should be a bit lower. Check the exhaust gas temp between the cylinders and exhaust manifold for each cylinder. Normal for a loaded non-turbo engine should be 300C. A cylinder temp lower than the rest shows a clogged injector or maybe a dead cylinder. A high cylinder temp possibly shows a leaking exhaust valve, or poor injector spray pattern.