At 60mph, 900 hours is 54,000 miles. But a boat under considerably more stress than a car. The answer depends on maintenance and use. There's no reason why a boat engine can't go twice that, but some people (myself included) have killed them in less than 1 hour.
Do a compression and leak down test. Chances are, if it made it 900 hours, it'll make it 100 more, which is a lot of time on the water. I probably put 2 hours a day on my engine, and do 8 weekends per year...so it would take me 6 years to get to 100 hours. Considering that
It'll probably get around 3-4mpg at 40mph, 100 hours (4,000 miles) will cost you $3500 in fuel. A basic engine rebuild is cheaper than that.