40HP Mercury Water Pump Issue???

I have a 40hp Mercury and recently on a trip water stopped shooting out of the hole. I replaced the impeller and had better water pressure than I'd seen on the motor since I bought it.......Ran it twice and the second time out after about 20 minutes I lost a considerable amount of water pressure. I got home and put the muffs on it and took the cover off the motor and there is a little hole on top of the motor that was shooting a little water out from time to time. I don't know what that little hole is and I don't know what that means. Any ideas what needs to be done would be appreciated!!!

That is the little hole water was coming out of

I believe that is the thermostat.

I took it apart and found out it is the thermostat housing. Long story short I started tracing my water flow and found where it stopped. Put the air compressor on the lines and blew a little chunk of grass out.....problem solved. Thanks for the info!!!