I took boat out yesterday to test engine, as boat has been sitting about 3 years. I got on plane then constant horn went off so I shut it down. Once engined cooled it would start up but had no power. Water was coming out of witness and i pulled thermostats today and tested in hot water. Anyone have experience with this issue?

do a compression test first thing. did it have power before it got hot?

I agree with TLW, do a compression test first. If that checks out, then immediately replace the impeller (water pump) and the lower unit oil. Then drain all the fuel and replace the filters. If you had fuel with ethanol in it, it goes bad and separates after about 6 months.

Sounds like the impeller (water pump) was bad and caused the motor to over heat, that is why I change my impeller at the beginning of every year. The Merc's have a very small impeller to begin with, so changing them often is a must. Letting one set 3 years is not good at all, they will normally weather rot and break apart or get weak and will not pump enough water.

You most likely will have to get the fuel system serviced from setting. Replace the water pump.

Engine did have power before alarm. I will replace impeller and check out fuel system. Thanks for the advice guys! Will the guardian system automatically reset?

do tha test before ya spend any money on parts!!! there is NO guardian mode on that motor....

Also check for water leaks under cowling at heads, t-stats and hoses.

Yes, do the compression test first before spending any money. Takes about 10 minutes and if it has a bad cylinder, will require a complete rebuild which will be expensive.

If needs a rebuild, let me know I have a friend who will either rebuild it for you or buy it from you. But hopefully it not that bad.

I changed an impeller myself on a 150 efi by watching a video on YouTube. Whole thing took about an hour and a half. I'm not the most mechaically inclined guy in the world either.