I love it when the only answer you want to accept is the one you have given,,, a cone shape with a wide top and a heavy ballest on the bottom removes the complexity of making a form with a r= 25 ft. An inverted “wedding cake” would allow you to build rings and stack them up, adding more rings would give deep water stability and shortening the stack would improve the draft. The lower rings need not be water tight, they canbe filled with ballest later to adjust the profile above the surface and trimming. An outside shallow ring and an inside deep ring could be joined by forming and pouring in the water… BUT the best way to for the 1/2 ball is to float the form IN the h20 and shot crete it like they do when building a water park…this allows you to eliminate the inside form and you sink the form to launch. Form seperation from the ball is done with compressed water… very low psi thru fill ports in the bottom… . The way to solve a problem is to look at it with an open mind.