5.7 ltr engine knock

Hi,
I have a Searay 250 sundancer with a Mercruiser 5.7ltr V8. When I was out on the lake travelling at 20 knots the engine bay filled with smoke from the breathers, I assume. The engine got noisy and now there is a knock, only under load. I stopped when it happened and I went back slowly after letting it cool. The engine didn't over heat and the oil pressure is fine. There was no knocking on the way back at 5 knots.
I let the engine fully cool when I got back. Then started it, no knocking. I let it warm up and gave it a few revs, no knocking. So I took it back out, at up to 6 knots it was fine, but as soon as I put a load on the engine it started knocking. It sounds like a bearing, but I thought it would knock on a cold start, it doesn't.
Can anyone shed some light on this before I lift the engine?
Could it be bearings? Water coming in the intake manifolds? Detonation? Or something else?
Thanks in advance!

Hard to say from here but if it is knocking with engine rev then it can only be bad. Sorry, no real help but the first thing to do is get the poor thing to a mechanic for diagnosis

I have a job where the engine started knocking but without all the smoke. Turned out a piston shirt had seperated from a piston. No water or oil contamination just a loud metallic knock. good temp and oil pressure,good compression.Sounded almost like a busted con rod hitting the oil pan
I`ll have pics shortly