Aft is the direction towards the back of the ship. Check out the nautical terms, I think it's in the sticky posts.
I just booked my first cruise, but as I understand it, you're only likely to get motion sick on a ship in rough weather, generally these ships are very stable motion-wise.
But my many years of experience in a sailboat has taught me that every one can get motion sick at some point and which motion varies by person. With some it's the side to side rolling, with others it's waves that come from the back in long intervals. So it depends on what the motion is and what you can do to correct it. Closer to the water line should have less motion if the ship is rocking. YMMV.