In Divinity Original Sin 2, when you progress from Act 1 to Act 2, you can't go back to the old areas.
I really really hope this won't be the case for Baldur's Gate 3. Early Access has so much content already. I've clocked over 90 hours now, and I still find things of interest.
It would be a shame if we are locked out of this content once we step onto the boat.