Exhaust

HI my son's boat is a formula it has 2 engines we notice this weekend while doing some electrical work that when we went to start the engines after hooking up the outside water hose to the hose fitting above the salt water strainer that no water came out the left hand exhaust pipes before starting the engine, Normally water comes out both exhaust pipes. Hooked up the hose to the right hand engine before starting and had plenty of water coming out. Hooked the hose up again to the L H engine and cranked the engine within several seconds of the engine starting water came out the exhaust as normal Any idea why this just started, Is it safe to start the engine without water coming out the exhaust first. Any thoughts would be appreciated Thanks Gary

Perhaps the lay of the impeller inside the raw water pump housing caused a partial blockage?? Did it only do it one time and not since? The sequence should be water on start engine stop engine water off.