I have not been involved with the system lay-out, but I can see the allure. Landing at sea helps, because:
- The population density is lower, i.e. it's easier to show that your landing doesn't endanger the un-involved public.
- Your landing point is relatively free, so you can return from varied orbits without needing excessive cross-range.
Why would you want to land _on_ the support ship/barge as opposed next to it?
- Dunking stuff into sea-water is harsh on said stuff. It can be designed around, but it constrains choices of materials etc. While solving an already hard problem additional constraints of these sorts are best avoided.
- Ships can be built in ways to almost eliminate wave-action, which would impose more (and more varied) structural loads in a water landing.
Is all this worth the trade-off of having to pull of a precision landing every time, and having to maintain/operate/design an (expensive, possibly custom) support ship?
I don't know. Mr. Bezos might know more, but I doubt that he knows for sure either.
Cheers,
Michael