level of the water in plastic 55 gallon barrels

Hi,

My pontoon boat has 55 gallon plastic barrels with lids. I will be sealing the lids tightly and do not want to open the lid to check if any water has leaked.
Are there any good ways to check the level of the water in the 55 gallon barrels without opening the lid ?

 

Typically, when the boat is pulled, the water in the barrel would be a different temperature than the air temp and condensation can occur.

In the water would be much harder without inspection ports, unless the boat lists.

 

There are many types of electronic moisture sensors and water level sensors available, if it is indeed worth the effort of purchasing and installing on an obviously low budget craft.
I’d mark the waterline at first launch, and monitor it for changes.

 

How about placing a small suction tube through the top of each barrel?
A quick puff of air through the tube will bubble if water is present, and can then be hooked up to a shop vac to withdraw water.

 

How are your barrels oriented, vertically?
If so, why seal and vent them?