If you are visiting Gros Morne, you should almost certainly do this. The boat trip was very nice, the scenery was very pretty, and the people on the boat are fun and entertaining.
But there are certainly things to be aware of. You have to hike to the boat, about a 45 minute hike. Make sure to bring bug spray for the walk, the black flies were out, but we did see a moose on the hike back. Even if you don't do the boat, it is a very nice hike.
You can buy your ticket in Rocky Harbour in advance. You can also do it at the dock, but I think it is cash only there. There is a nice stand at the dock, with bathrooms and some limited food (hotdogs and soda). If you want more, pack a lunch. Be aware, they do advice you that if it is too foggy then can cancel the boat trip, and they will exchange your ticket for another day - but if it is your last day of the trip, that might be a problem.
We didn't see any animals from the boat, just some very pretty scenery - rock formations, waterfalls. If you want something more, this might not be your trip.