Is Mljet National Park better as a day trip from Dubrovnik or from Korčula?

Usually from Korčula, because the park is slightly closer and the day tends to have less transport friction. Dubrovnik still works well when the sailing matches your stay.

Can you do St Mary Islet and the lakes in one day?

Yes. In fact, that is the most sensible first-time structure. The official ticket already includes one return solar-boat ride to St Mary.

What is the easiest route for a first visit?

The official Pomena route is the easiest: promenade walk, Malo Jezero, Mali Most, then the solar boat to St Mary.

Can you enter the lakes with your own boat?

No. The official FAQ says entry by boat into Soline Channel, Veliko Jezero, and Malo Jezero is prohibited except for vessels owned by local residents of that area.

Can you enter the lakes with your own boat?

No. The official FAQ says entry by boat into Soline Channel, Veliko Jezero, and Malo Jezero is prohibited except for vessels owned by local residents of that area.