Depends on the gearing and the prop.
It's all about "thrust"!!
An 8" dia. prop with a 6" pitch
and your 6hp doing wot, you will punch through heavy seas and wind on the nose
with the best of anyone.
IIRC, at the time I had my boat there were no "sailboat special" outboards with extra long shafts and high thrust props and gearing. I used the largest diameter prop with one inch less pitch than the stock prop. It worked fine, with the 15HP I ran mostly at about 3/4 throttle. Full throttle in high short chop pounded the boat pretty badly so I seldom did that. The important thing is to keep the prop in the water in rough conditions regardless of engine size.