Mary, have you looked closely at alternatives to the ferry....cheap flights?
(Caveat: I have not been to Corsica yet, but I've been looking at a trip there for some time, it's near the top of my list, and I've been researching options to get there and back.)
You can fly from either Marseille or Nice (both are pretty close to Nîmes) to multiple airports around Corsica. There are short, quick nonstop flights to all 4 airports around Corsica (Bastia, Calvi, Ajaccio and Figari) from both Marseille or Nice (both of which are pretty close to Nîmes). And those flights are cheap -- all under $100, most priced between $57 and $72 (as long as you buy tickets well in advance). Check out Air Corsica.
Where are you headed after Corsica? There are also flights to many larger cities around Europe, so once you've enjoyed your time on Corsica and it's time to move on, it's easy and pretty inexpensive to fly to many cities -- point being, taking the ferry back and then moving on may be a waste of both time and money (compared to flying back from Corsica to your next destination).
Personally, while I enjoy a nice ferry ride as much as anyone, unless you really have a lot of time to kill or just hate flying, I'd take that short, cheap flight. You will save a lot of time, and the flight tickets might not cost much more than the boat.
Can't predict how rough the seas would be on any given day, or exactly how prone to motion sickness you are (my spouse is very so I am familiar with the concerns). The flight will be brief, the boat...might feel longer than it actually is. These days with cheap air tickets, I usually find it works out much better to simply fly.
Hope that helps.