overheating piston damage???

after initially having no water peeing out of my 1980,7.5hp mercury,i changed impeller,still no water.then opened up the exhaust cover( sheering 2 bolts in process) on inspection found the water inlett clogged and blocked.after cleaning and degreassing the are i notice there are 6 holes.i think these are some sort of inspection holes.ican see totally smooth sphericle what i think might be the the pistons?? however through one of these holes and only through one the shiny clean piston if that is what it is, seems to have 3 paralell lines scored in to it! im begining to worry that my pistons are damaged through excessive heat.also found that there was no thermostat fitted.

You are likely looking through the exhaust ports and are indeed looking at the pistons.-------Those lines are not a good sign and your motor will run but not well.-----------------Time to tear it apart for overhaul.-----------Before you do that check the availability of parts as many new parts are now NLA as in no longer available.