There is not such thing as "good enough" when it comes to boating. Don't settle for anything less than the maximum HP it is rated for but do not go over that because if an accident occurs, you are liable for having an unsafe boat by overpowering your boat and it will be treated as such by insurance companies, lawyers, and jurys.
With the max. HP, you can get on plane faster-saving fuel, run the engine at 1/2 throttle and get better fuel mileage than a lesser motor run WOT going the same speed.