Mercruiser knocking/overheating

I just took my boat out for it's first cruise today. It has a 98 Mercruiser 5.0 inboard/outboard. I had the boat running out of the water several times before putting in the water. The water flow through the cooling system was great and the motor was running smooth. About 15 minutes into our trip the motor started knocking so I stopped and opened the engine compartment. The motor was smoking slightly, smelled like burnt oil and was very hot. I pulled the dipstick and it appeared dry. There was no oil in the bilge or around the boat. I had the oil and filter changed when it was pulled out last year. It took over an hour of checking the dipstick before oil appeared at the full line of the dipstick.

Why did the oil take so long to return to the pan?

Does this seem like a problem with the oil pump?

Re: Mercruiser knocking/overheating

Were you running at WOT when it overheated/lost oil/whatever happened? What did your oil pressure gauge or engine temp gauges say during this episode?

I'd take it home, put it back on muffs, fire it up and check it all again.