[jws]

Best case, they get to test the new sound dampening, armored pad, armored pad cooling, igniting all the engines on the pad, booster launch, staging, booster fly back, booster descent, booster landing (though not on anything solid). If that works they have a booster floating in the Gulf of Mexico they need to clean up.

For Starship they get to test staging, engine in vacuum, bellyflop control from very high altitude to sea level. Apparently all the way to sea level. They do not appear to be planning the reignition and flip. They may set the record for "most powerful bellyflop".

All the excitement is in the first 9 minutes of the flight. A little over an hour later the Starship's "orbit" will intersect too much atmosphere and will re-enter for about 12 minutes.

As for why not to try landing the Starship on water, I think they'll play with all their available fuel mass in vacuum. Those engines haven't operated there yet.

[extraduder_ire]

I'd be surprised if they haven't tested at least one engine in vacuum. You can build a test apparatus for that. Only learned how when I saw this video: https://www.youtube.com/watch?v=hrLyzpTV7GU