Desirable qualities of a launch site:

 1. Near the equator, to take advantage of the Earth's rotation speed, allowing a very slightly greater payload for the same cost in fuel weight
 2. Good access to rail and/or port, because big rockets require lots of big pieces to be brought in for assembly
 3. Acceptable safety zone, because are slow burning bombs that do not always politely burn slowly

Thus a floating platform is arguably an approximately optimal solution, which is why SpaceX does this.  NASA (Florida) and ESA (French Guiana) fit the pattern.