I have noticed you posted in several other areas this same question. There are a ton of very knowledgeable people on this board, but we need a lot more info to help you out.
First of all, what kind of boat? What is the intended use? Are you rebuilding the motor, or just looking for a lumpy idle or a little more performance. What kind budget do you have?
These are all important pieces to selecting the right cam. It really should be a package, with exhaust, intake, carb, heads, etc. to get the most out of your upgrades.
Unlike a car, just throwing a new cam in for a few more ponies and a throaty idle, does not work too well in a boat. 20 more HP in a car is noticeable, in a boat you might see 1-3 mph depending on a lot of variables.
The Merc 5.7 packages are well engineered for the boats they were used in. You might find that leaving the motor stock and simply changing props will give you the performance you are after.
Welcome to OSO, give us some more info, and we can help you out!