just rebuilt twin mercruiser 500 EFI. starboard engine water jackets plugged with mud blown head gasket, torched head and block. Didn't have a clue till performance fell off. The temp gauge stayed at 150 deg and water pressure was about 10 lbs on both engines. How are you supposed to know you have a problem? Had both engines rebuilt to 600 hp. new alum heads, new cam with the latest corvette cyl swap, new lifters, rockers, pistons. Installed sea strainers to prevent the same problem from happening again. Well it happened again 300 boats at a party and went through the same channel and I sucked up some sand. After leaving and cruising about a mile or so the port engine heated to 180 deg. I checked everything I could think of. Used the fresh water flush out. Replaced the water intake impeller (which was perfect) and it still heats up at idle. It cools to 150 at cruising speed, the same temp as the starboard. How am I supposed to know if I have mud in this engine and ruin it like I did my old one. I have built engines and raced cars my whole life and I never had one I couldn't flush out, or one that had no symptoms of a problem. The Ohio River is full of low spots even at some of the best marinas. These motors are in a 2001 Sonic 42 SS . Anyone have any thoughts?

Have you pulled the drain plugs and run something like a coat hanger in there?
Have you back flushed through all the coolers? (fuel & oil)

Also, are you sure you don't have a restriction somewhere?
Do you have water pressure gauges?
How about flow out the tailpipes? Is it the same on both tips?

Pull every hose one at a time and backflush.

Pull the block drain plugs and see if the block is packed with sand/mud.

Didn't pull drain plugs, (good idea). Back flushed starting at coolers, tiny bits of clean fine grain sand. Water pressure at 10 lbs. each when at cruising speed. I haven't been able to get to the pickup on the outdrives to check for restrictions.

i would pull drives and make shue that hose in gimble hose is clear also

The Ohio River sucks - I've clogged so many coolers with fine river pebbles, I've lost count. Flushing with a hose is not sufficient. You need to use a flashlight to ensure you can look through each tube in the cooler and use a coat hanger or welding rod to push the pebbles out.

As previously mentioned, once you've confirmed all of the coolers are clear - pull the drain plugs and ensure you have water flow from the blocks.

One way to prevent a lot of the sand and mud from getting in the block is to install some transom mount pickups. Since you are picking up water from the drive, it is the lowest point and will hit sand and mud first. If you have the engine running, the water pump is sucking on the holes in the lower. If you churn up mud or sand, it's going right in the pickup. There is no way around that. Also, there are no strainers out there that will catch sand or mud that can flow enough water to supply and engine. They have them for intercoolers, but they don't require nearly as much water.

A transom mounted pickup will help drastically. It may not prevent all of the sand and mud, but by raising the water's pickup point by about 2', it will surely help. I understand that you may run through shallow spots that are unavoidable, but don't beach the boat with the engines. Turn it off and walk it onto the beach.
Eddie