So why do they need to use helicopters and a risky airlift to return the astronauts to the main vessel? Why not just use the speedboats to take them back? Seems really odd and I can’t find any reasonable explanation.

>Buoyancy is an easier equation to solve than lift.

That's a snappy one-liner but it doesn't address the real concerns.

First of all, subsonic lift is well understood and has been for decades. The answer is much more mundane: The Artemis mission profile does not require payload doors that open, no Canadarm, no requirement to service, launch, and/or capture satellites in orbit, and so like good engineers they designed the minimum vehicle that satisfies the requirements.

Also, the Shuttle was actually much more expensive to reuse than originally predicted.