The Tonkin Gulf has different 3 parts: 1) Halong Bay 2) Bai Tu Long Bay 3) Lan Ha Bay.
+ (1) Halong Bay is the most beautiful.
+ (1) Halong Bay and (2) Bai Tu Long Bay which belongs to Quang Ninh province.
+ Cat Ba island on (3) Lan Ha Bay and belongs to Hai Phong province (different provinces; different state management). All the cruises can not go to Halong Bay if starting at Hai Phong or Cat Ba.
>>> Note: If starting at Cat Ba you can only visit Lan Ha Bay, not Halong Bay (Only has islands in Lan Ha Bay; no caves; no panorama from on the top of Ti-Top Island...)
For Halong bay tours which will take you to different parts of Halong Bay:
1st tour: Halong bay day tour with 4 hours of cruise (a half day from Halong City or one day from Hanoi City).
2nd tour: Halong bay day tour with 6 hours of cruise (a half day from Halong City or one day from Hanoi City).
3rd tour: Halong bay 2 days tour with Overnight (sleep) on the cruise (two days from Hanoi City).
>>> Read more: Best of Halong Bay Tours