Overheating alarm after sandbar

Yamaha outboard v4 115 hp overheating !!
The boat ran excellent at full throttle all day, to we went thru a sandbar, and it sucked in a bit of sand and the overheat alarm came on !!!!!!
The outboard didn't perform after that, limping home with revs not exceeding 2,000rpm unless alarm will come on ?
I'm going to try a few things to rectify the overheating, that i've read from previous forums.......
Is it dangerous when flushing a Yamaha outboard on rectangle muffs, to put it in gear and rev it past the 2,000rpm to see if it still overheats after trying to rectify a few things ????
Thanks

you need to drop lower gearcase and changeout waterpump,pull thermostats, Put hose on water tube,flush upwards,and flush downwards thru stats opening

So flush upwards first then downwards thru tstats second ??