Nah, the sea sickness is not a big deal to probably 99.999% of people. I would not worry about it. Crap, I might even get sick on one of those stink pot volmit (I mean fishing) expeditions!
The boating lifestyle is awesome, hard, expensive, and free all in one! You get to pick which one you want to focus on. I seem to do a bit of al of them. We have two kids on board, 6 (now) and 9. It is easy with them as the age is right. However, they have been on boats most of their lives. I am not sure how a teenager would make the transition. Everyone here will give you an opinino on that, but only you know your child and their temperment. I will say that if they are engrained in friends, it could be hard.
There is a great book by Tom Neale called 'All in the Same Boat'. Get it. It is cheap and a grea read. I am a huge proponent of his thoughts and they mirror mine. At the very least, it will give you a feel for what to expect. He raised two girls on the boat.
If I can help answer any questions, let me know. However, my viewpoint is tainted as I recently sold everything to do this and make it happen. And my little sailors take sailing seriously, see....