The boats center of gravity, LCG. BUT, by being at the LCG you are still going to move in all three dimensions quite a bit as well, proportionately more than the difference between being at the LCG and out on the very bow or stern of the vessel.
Look up tips for avoiding seasickness. I think some of those are going to be the greatest benefit to you rather than just getting yourself as close to the boats center of gravity as you can.
Mark is right about moving weight towards the CG. This reduces what is called the angular momentum of the boat. You get a faster pitch
and roll rate but less of it. Because of their relatively low buoyancy bow and stern (compared to a monohull), cats can "hobby horse" quite a bit with unnecessary weight at those places.