Not neccesarily.How high do you plan on revving your motor?A bigger cam will raise your hp levels at higher rpms.do you want to be revving 8k to make power?
A cam that is to big in a jet boat can hurt you.If you are running an a impeller you will probably not see high rpm for that cam to work.you might need a smaller impeller.
If you have a high comp ratio,head combo carbueration,etc, and everything to match then you can go with a bigger cam.
How big a cam are talking about? This all depends on what your current set up is.If you want alot of reliability stay with a hydraulic type.If you are running a near stock motor with mild or no head work stay with something aroun 230-240@.50 duration with around a 550-560 lift on a 108 center.this should work well for an a -b impeller which should put your peak hp around 5-5500rpm.
A hydraulic roller type would even be a better choice if the funds allow the big advantage would be a motor that will make the same hp at higher rpm and more torque at lower rpm with the same or more hp.