You probably sucked up sand or debris and you should check your strainer and clean it and back flush the raw system from the front of the oil cooler back and check afterwards to see if the system is pumping a good flow of water out the exhaust or read the water pressures with a scan tool. If you don't have a raw water strainer-SHAME ON YOU, especially if your going to boat in shallow water areas or debris filled water. The pressures should be 2-5psi min at idle and should climb to 12-15 psi at 2500 rpms or so. If not then you may have wiped out the raw water pump impeller and housing if sand was the problem. The "guardian" alarm generally goes off about 185-187 degrees and its ok to generally idle and low rpm ("guardian" system limited) run until the engine gets to about 195. Beyond that save the engine and go for a tow, its still less expensive that rebuilding an engine or replacing blown head gaskets!
Hope all is minor and easily fixable.
Ray @ Raylar