We did the crossing in springtime, but the weater had been really bad for a few days. We were told by numerous people "you'll have a rough crossing" and we didn't really know what that entailed.
First of all, when we got to the ferry many of us were surprised that it wasn't really a ferry. You know, the kind that carries cars and trucks. It's more of a boat. It only carries people, and luggage obviously.
Our journey started off smoothly, but very quickly turned into a rollercoaster. The waves were massive and at times we felt like we were flying. A few of our group that tended to get motion sickness were not happy, but the staff were very helpful in suggesting what to do and handing out cloths of cold water. Even people who don't normally feel sick at sea were feeling a bit rough by the end of the 1 hour journey. The main thing is that the staff is friendly and helpful.
Our crossing to go back the next day was much smoother and even the people prone to motion sickness felt fine.
I think it very much depends on the day of crossing so you can never be sure what kind of waves you will get.
Forget about doing a bunch of extreme sports in NZ, just get the ferry on a windy day. That's all the adrenaline you'll ever need!

The crossing from Bluff to Stewart Island was very comfortable and peaceful but it was a different story on the return journey when the seas were rough. The skipper was very experienced but it was pretty scary at times and it certainly made me feel queasy despite taking a seasickness tablet before. We were later told that this stretch of water, the Foveaux Strait, is one of the most dangerous crossings in the world so I certainly wouldn't be doing it again in a hurry!

Only had coffee at hotel.
Others in group raved about there blue cod meals....
Friendly staff
Right on beach front
You look out of resturant onto bay.