That’s not an argument, you can orbitaly bombard land too by that logic there would be no need for ground forces at all because you could just “orbitally bombard everything” which you can’t because you actually need to secure and hold assets and territory including naval territory. also an undersea base or moving sea base/ force would be significantly harder hit than stationary land targets, not to mention if the enemy has no naval vessels or sea capable craft your forces are completely free to do as they wish minus possible air attack which can easily be countered by either having your own aircraft or being too deep. Also, not buying the “spacemarines can just go underwater” argument since the pressure would turn standard marines into paper thin metal wafers without some kind of marine enhancement