This is a "SeaCore" catalyst engine. They have a fairly complicated cooling system. These engines generally require a dual water pickup.
"Models operated above the 50th parallel of the Northern Hemisphere or below the 50th parallel of the Southern Hemisphere do
not require the dual seawater pickup with a Bravo sterndrive on closed‑cooling models."
"SeaCore Models
Some SeaCore models do not require a through‑the‑hull or through‑the‑transom seawater pickup to meet the minimum flow
specifications.
The SeaCore Bravo One and Bravo Three engine packages do not require a through‑the‑hull or through‑the‑transom seawater
pickup in addition to the sterndrive water pickups if:
• The sterndrive gearcase has dual water pickups.
• The boat is capable of 64 km/h (40 mph) with the boat fully loaded and operated within the specified operating range.
SeaCore Bravo Two Models
The SeaCore Bravo Two engine packages must have a through‑the‑hull or through‑the‑transom seawater pickup in addition to
the sterndrive side water pickups. Install the Y‑fitting at the engine's seawater pump inlet. Refer to Installing the Y‑Fitting."
All this information is covered in MerCruiser Manual #51 p/n 879288300
So it looks like you have 3 options;
1. Do some of the things listed in the posts above --- compression test and new impeller and hope you get lucky and it's something simple.
2. Get a copy of manual #51, read and check that your installation meets all the requirements and then follow the troubleshooting to find the problem and fix it.
3. Take it to a mechanic who has read the manual.
And I can guess what you're thinking right now because I've heard it so many times from customers.---- "But until I did this one thing (in your case ran aground) it's always worked fine. So I'm sure the problem is just because I did this one thing" ---- But unfortunately I've found that's not always the case.
MerCruiser sets many rules when it comes to the installation of their product. Static water-line, compartment ventilation, minimum flow specifications and many more. These are usually followed pretty closely by the boat builders. But the small shops and dealers when they do a repower, not so much.
What I've seen is that those rules can be "bent" a little when everything is brand new. New impeller, new battery, new and clean poppet valves and springs in the cooling system, etc. etc.
And now along comes a minor thing (like maybe your low water pickups are plugged) and it shows that the install was never fully compliant, which turns it into a major problem. And overheating on a catalyst engine can be a major $$$$$ problem.
