I prefer Nassau because of Atlantis, an elaborate, upscale resort that includes a huge waterpark, aquarium, beach and casino. You can either buy a day pass for the water park & beach, or book an excursion via
DCL when they're offered (they typically are offered, but not since Covid- day pass is cheaper, anyway). Atlantis Aquaventure (water park admission) also includes access to their beautiful beach- it's all quite impressive, but not cheap.
No, there are no walk-off-the-ship-to-the-beach options on DCL's Eastern (or any other) itinerary. That kind of convenience is why private islands exist. An excursion isn't needed, though (unless it's due to Covid restrictions). On the major islands of the Bahamas and Caribbean they always have shared taxis that take tourists to beaches and other attractions. You just walk off the ship, and will almost immediately encounter the taxi dispatching area. Tell the dispatcher where you're going, and they'll put you in a shared taxi. Be prepared to pay cash- it's usually around $5 per person but it varies based on locale and destination (have extra cash handy, especially singles). At the destination, there will be another dispatching area to take tourists back to the port. It works like clockwork for the major destinations such as the ones I'm describing in this post.
On St. Thomas, Megan's Bay is a draw for families because of its calm waters. On Tortola, Cane Garden Bay Beach is popular. Personally, I thought both beaches were okay but nothing awesome- the public islands that DCL visits are pretty run-down and impoverished. The beaches themselves are clean and pretty. Just don't expect to feel immersed in a tropical paradise- that's also what private islands are for.
One thing you can walk to on St. Thomas is the Skyride to Paradise Point tram. At the summit, you'll get a great view of the island and the ship. It's a short, easy walk through a safe area to get there from the ship. On Nassau, you should not walk around town, for safety reasons. Stick to taxis.