Thanks folks. My comment about outboards not being as highly stressed as marine diesels comes from a comparison with road engines. For instance, the Volvo D12 can make 775 hp whereas the road vehicle engine it's based on only makes 440. The Cummins 6BT in a boat can put out 425 whereas in a truck it's only rated at about 260. The Volvo D4 can generate 300hp in a boat yet probably only in the region of 190 in a truck.
Outboards on the other hand are not, I don't think, widely based on vehicle engines. A Mercury 50 is a 1000cc 3 cylinder motor. The Vauxhall/Opel Corsa also has a 1 litre 3 cylinder engine that puts out 55. An Evinrude 90 ETEC is a 1.7 4 cylinder 2 stroke, but a typical 1.6 llitre 4 cylinder 4 stroke car engine easily develops 100 to 120bhp.
I realise this is not comparing like with like in terms of load, drag and wind resistance, but I am thinking in terms of displacement and the origin of the engine.
It would seem from your replies that cost is a major issue, and I presume space is also as an outboard doesn't have the same amount of space as a car. My thoughts about a gearbox with more than one ration though wasn't so much about performance as fuel consumption and longevity, I did think that reducing revs via gearing might increase the life of a motor. Mind you the more I think about it, the more I realise that, with the exception of commercial applications when a diesel might be more appropriate, outboards don't really work the hours that road vehicle engines do, so I think I might have answered my own question there.
With regard to the Mercury Verado, I must admit that output is pretty impressive, but Yamaha and Suzuki both have 1000cc bikes that produce something in the region of 175bhp which if you multiply it by 2.6, equals 455hp without any kind of forced induction.
Thanks very much to everyone who replied. Sometimes I like to know the ins and outs of a f**t, but overall it helps my understanding of the subject. The comment about reduction gearing was also useful as I wasn't absolutely sure whether it was reduction or multiplication.