No matter how you do it, the same kind of variables apply
You can move the laser around, but that still doesn’t give you a distribution of the heat within the mass of the water. If the ‘lid’ is open, you lose heat. If you stir it, you lose heat to the stirring device. While you wait for it to distribute, you lose heat. How much heat is directly related to the temperature difference between the surfaces, the more difference the more heat loss.
Simple fact, if there were a good, cheap way of doing this, it would probably be all over the Internet and all of us would be using it.
Cheap, simple way, buy a crystal ball and believe it… 
There is the Mahoney meter
https://bell-laser.mybigcommerce.com/mahoney-co2-laser-power-meter-probe-0-100-watts/
and the dohickey from Russ.
Best options that I know of…
