Penta 8.2 GSI raw water cooled restriction issue.

2000 8.2 GSI, I live on a shallow muddy bay and a few weeks ago didn't realize my trim cylinders weren't working so dragged my drive through the mud leaving my bay, sucked up a bunch of crud, overheat alarm sounded so i went back in. After rebuilding the trim cylinders it was still getting hot so traced the raw water intake through the water pump, then goes through 2 small heat exchangers for the steering and something else before hitting the thermostat block where i was getting no coolant flow (Raw water pump has a new impeller and was pumping good) so i disconnected at the thermostat housing and back flushed back to the pump outlet and a bunch of mud came out. I replaced the thermostat while i was at it and on testing everything its operating at 150 degs or so and i can see the thermostat opening and closing by the gauge needle, can hold my hand on top of the risers though the port side is warmer and i have water spitting out of the through hull exhausts and all seems well. So here (finally) is my problem, I open the drains on the block to verify good flow cooling the engine and get hot water flowing out of the one starboard drain, but on the port side where there are 2 drains, i get hot water out of the lower one but absolutely nothing out of the upper one. Poked a wire in there and its clear, tried back flushing it with a garden hose and don't get any flow going into it either. I'm worried part of the block isn't getting cooling water and will overheat and it wont show because the rest of the engine is cooling. Should coolant come out of the upper block drain? Appreciate any advice.