This is basic thinking but maybe worth a try. If you have your 30 for PC on a lift or can get it out of the water open the seacock and remove the strainer. With a stiff wire or a fish tape snake the hose from the strainer housing to the seacock. Next use a water hose to blow the hose and water intake clear. It helps if a second person can tell you if the water is exiting the intake without restriction. Remove move the hose at the water pump coming from the strainer housing. Sneak this hose out from top to bottom. With a couple of quick blasts of water in the hose watch the strainer housing again for debris. Put the cover back on the strainer housing leaving the strainer out. Again with the water hose under pressure flush the hose through the system until it exits the hull without restriction. Replace the clean strainer and water pump hose. Even if this does not work it will eliminate any doubt of restricted water flow. Good luck
Does it run, then shut down a few seconds later...throwing the cooling code? If so, was there plenty of water coming out of the side discharge? If all of this, it's probably the cooling water flow sensor that inserts into the heat exchanger. If it senses lack of cooling flow...or if it is defective....it shuts down the genset. Unplug one of the two plugs on the sensor and start the genset. If it runs, replace the sensor.