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-beerjet-

Depending on what you plan on using the boat for will also determine what cam would be best for your application.

Assuming it'll be a typical river racer hot rod and you don't plan on pulling a tube or skier much I'd recommend the Comp Cams 11-702-9 if you want to stay with the mechanical roller.  It's a decent size cam without being so big that it's hard on parts.  It gets expensive to replace valve springs every season or more depending on how much use it's getting.

Now if you plan on drag racing or trying your hand at roundy-round I'd go with something a little different.

Like crewchief is saying, it all has to work together, from the air coming into the carb, to the spent gas exiting the exhaust, carb, intake, compression, exhaust are all part of the equation.  Each piece helps pick the next piece.   Not knowing what your heads will flow, it's hard to suggest anything.  Then where do you want to make power at in the rpm range, above 4000 or done by 5000?  Sorting all this out ahead of time will get you  the results your looking for instead of people just guessing at what they think you need.  Have as much info and specs as you can on what you already have and then contact someone like Clay Smith cams, or Chris Straub Technology's.  These guy's will crunch the numbers and get you exactly what will work for your build.  Other people, like the local speed shop are just guessing.  If they're are not asking you these questions, then they don't know what they're talking about. JMO.  Good luck
http://www.straubtechnologies.com/