Thanks for taking the time to visit our forums. This is an interesting topic, and one into which a lot of time and energy was spent early on at the Institute. Check out the seasteading wiki: http://wiki.seasteading.org/index.php/Main_Page – specifically the sections under Structure Designs, Spar, Stability, and Low Cost Materials.
You’re probably already on the same page as a lot of thinking – such as minimizing surface area closer to the surface, where waves will have the most force, and having a heavy ballast to lower the center of gravity. You might also want to look into Gyroscopes, space frame, and the conex dumbbell.
I’m curious to know what you think of all of this!