See a few posts back for seaworthiness. Seaworthiness has something to do with stability and stability is aided by a low center of gravity. But heavier boats glide better as well because they have greater momentum than a lighter boat at the same speed. So we're hoping that you can make your kayak heavier and therefore more seaworthy. The Aleuts reportedly carried ballast in their kayaks, ballast equaling roughly the weight of their kayaks.
So today I went out in my Aleut kayak and brought along one of those three sandbags pictured above. These guys weigh about 35 pound apiece. The idea was to place a sandbag in back of the cockpit to see how that would affect handling of the kayak. First of all, 35 pounds is fairly heavy and hard to push around in a boat with ribs. 15 pound sand bags might be a better choice. Not only is 35 pounds heavy, but it's also too big and hard to get into a tight boat.
But on to the effect of 35 pounds on handling. As expected, a heavier boat is harder to accelerate. I was expecting the effect to be negligible, but instead, it was noticable. Glide improves, I suppose but paddling definitely takes more effort with a heavier boat. I had been hoping that the extra weight would affect mostly acceleration, but it affects cruising at constant speed as well. I suppose. Wetted surface increases as does the amount of water that needs to be pushed out of the way by a heavier boat. All in all, extra ballast on flat water is mostly an annoyance. Whatever is gained isn't worth the extra effort.
But I'm still holding out for improved performance in rough water. I'll have to give the sandbags another try next time the wind blows.