When you are crossing an ocean, you are forced to spend many weeks at sea in a small floating hallway. Every now and then you walk to the bow to enjoy the view or you make your way to the mast to raise or lower a sail, reef, or check for unlucky flying fish that have landed on the deck. It's possible that you will remain with the same sail plan and tack for days on end, however, and with automatic steering, crossing an ocean becomes napping, reading, or watching for whales on a bobbing platform for a very long time. This is not exactly conducive to an active lifestyle. Coastal sailing is hard work, but crossing an ocean requires very little actual physical activity. It is therefore very important that you make an effort to exercise in some way while you are out. I found it very difficult to force myself each day to try and keep my muscles from inevidibly depleating. I often turned to yoga since it is easy to do in a small area, but it is a practice that is rooted in stability and that can become a problem if the seas are at all choppy. Both of us lost a substantial amount of weight on the journey and I believe it was more from lack of eating excessively than anything else, but we definitely lost muscle because it is almost impossible to do the amount of conditioning necessary to maintain what we had when we were running and hiking. I recommend planks. Your core gets a nice workout from simply stabllizing your body throughout the passage, but planks are great for many additional muscle groups including legs, shoulders, and back. It is easy to do a plank at any point in the cockpit, on the bow, or below in the cabin. You will find that a few 30 second planks a day will do wonders. I tried to run in place a few times, but that was not great for the boat's surface and it was difficult to keep my balance. For legs, I recommend squats. Be sure to hold onto something if you have to, and stand on your tiptoes to include the calves. Leg lifts, crunches, and push-ups are all easy, low to the ground exercises that can be done on a boat during all weather.
I mention all of this because it is easy to forget to take care of yourself during an ocean crossing. This is because it feels like a fantasy. You are so far from the norm that you are used to, and you measure your time very differently from when you are on land. Everything slows down, and you can forget to eat, drink, and exercise because life has a different pace out there. Give yourself a schedule. Set a time to eat, a time to do a plank, a time to take medications or vitamins. Because the physical effect that cruising has on your body doesn't have to be immense at all.