larson engine seizing?

I have a 4.3 L. 175 HP engine on an OMC Cobra drive, 1987 Year, 19 foot Larson Citation 190 bowrider I/O. The boat engine stopped abrubtly at 3/4 throttle on a plane when I took it out last season. A local boat dealer said it needed new carburator which they installed. Same thing happened again this season, the first time I took it out after last year's repair. It happened just like before, about 5 minutes after heading down the lake. In order to narrow down whether it was an engine or transmission problem, I put the garden hose attachment on the lower unit and started it while parked on my trailer in my driveway. It ran only 30 seconds and stopped abruptly while not in gear. It sound like the engine is seizing up. I checked the oil and it looked full and Ok color without evidence of water or metal contaminants.

What would you recommend is the next step? Any guess as to the root cause of this problem and costs to fix? Who do you recommend to get any estimate and what would you estimate it is worth to sell the boat as-is?