welcome aboard.
where are you located? where are you boating? fresh water, salt water?
if your block passages are plugged, you will still get a good tell tale stream, however no cooling.
let us know what motor you have and where you are located. And if you are boating in Florida, and boat in salt water, yes, your block passages are most likely plugged and can only be mechanically cleaned (by pulling the block covers off and poking the crud out)