A solid alarm (horn) on the Opti indicates either an issue with the oil system or cooling system (in either situation the Guardian system will restrict the rpms as you note to prevent "significant" damage).
The Optimax is a complicated motor - there is almost nothing else out there like it - it just doesn't work the same way as traditional 2 strokes or 4 strokes.
If you plan on working on this yourself this is one of the cases where I absolutely suggest the "more expensive" factory service manual is in order (as compared with Seloc's or Clymers which is often sufficient for the average outboard).
Then, if you have not serviced the waterpump in more than a couple of years I would start there - new impeller, gaskets, seals as required.
The cooling system on this is also different from the average outboard. Just because you have a good tell-tale (pee stream) it doesn't mean that the heads are being cooled. You might have enough water pressure to get the stream but it may not be getting to the heads. Or, the thermostats may not be working properly, or the poppet is fried.
IF you take this into a dealer they will plug it in to a DDT and get an instant answer as to what caused the alarm. Yes, they may charge somewhat over 100 bucks for this piece of information but it could be cheaper in the long run than simply trial/error part swapping to fix the issue.
When you own a "state of the art" motor you sometimes give up the ability to fix it in the driveway with a wrench and a screwdriver..
- but just google Mercury Marine. Merc's website will give you a list of accredited dealers in/near your zip code...