The sea based concept has already been done, it's called "Sea Launch
". Prime is Boeing. They use a Russian rocket for low cost, and they've done ok, but had a few missed launches and my understanding is they're in bad shape right now. The rocket launching industry is extremely competitive with Lockheed Martin and Boeing merging now to create the United Launch Alliance
to help reduce costs. Pegasus is of course, only for very small satellites, so small they can't even be counted.
I'd agree with Mech Eng, the size of the balloon is prohibitive. There have been companies such as Skylifter
that have tried to use balloons for very heavy lifting, and they really aren't economical. The problems with operating a balloon that large include enormous costs for helium and helium clean up, ground facilities, and I suspect many other issues I'm not aware of. It's just a very impractical way of lifting a large payload.