I'll throw this into the inevitable list of messages that's sure to follow:
Every boat that takes an outboard motor will have a max suggested
horsepower. You may not need that. Try and find a dealer who seems to care
about what he sells, and find out how much horsepower your chosen boat needs
to simply be safe, taking various factors into account. If you don't feel
you're getting an intelligent response from the dealer, call the boat's
manufacturer.

Above all, the boat needs to be able to manhandle the kind of turbulence
you're likely to face on a bad day. Get you home safe, in other words. To be
fuel efficient and comfortable, it needs to be able to plane and still have
a bit of reserve left while carrying a load (people, gear) you feel will be
typical. For instance, my 14' yacht has a recommended max of 25 hp, if I
recall correctly. I'm sure the main factor is the weight of the motor. But,
I have a 15 hp motor and with rare exceptions which don't impact safety, I
never feel I'm missing anything.