Sound ok
By cold water discharge, I am assuming you mean the exhaust pipe. This is a boat specific sort of thing. Depends on the actual level that the muffler is at in relation to the water line and any interveining hoses, how the boat is loaded and how fast the engine is running. With that said, 10-15 seconds at idle is on the long side but as long as water is comming out every 10-15 sec then you have cooling water flowing. You should be OK but watch it while under load to get a feel for it. Then when something changes you can catch it quickly.