Typically, the Yanmar engines do not have a full set of gauges.
That warning horn, actually a Sonalert, is like the "idiot light" on most modern dashboards. It means something is wrong.
Suggestions are either oil PRESSURE, or engine is over temperature. The easiest way to check pressure, is to first change the oil filter. Second, verify that the oil level is where it needs to be. Third, unscrew the pressure sender, and connect a pressure gauge where the sender on the engine block was. You should see over 5 psi, and less than 40 psi. The pressure should vary with engine speed.
For over temperature, first check the impeller on the raw water pump. Make sure that all the vanes are there, and not cracked. Second check the level and condition (specific gravity) of the engine coolant (freshwater & antifreeze). If the impeller checks out, and the coolant is OK, then I would check the engine with an infrared temperature gauge. Another member here, Maine Sail, has written an excellent overview of what to look for on a Westerbeake engine, but you can apply the same theory to your Yanmar. Here is a link to that writeup; Engine Temp Monitoring & Over Heating Baseline Assessment Photo Gallery by Compass Marine at pbase.com
If these checks don't identify the problem, it is time to call a mechanic.