In the Northern Vietnam does not offer much blue water and diverse range of water-based activities. The coastal area in the North may not live up to your expectation if you were once to Nha Trang or My Khe Beach. However, if you really crave for some salted water and open air, they will do the job.  
Kindly look at the 5 best beaches near Hanoi as follow:
1/ Bai Chay Beach (Quang Ninh)
Bai Chay Beach is located in Quang Ninh Province, around 100km to the Northeast of Hanoi. The most interesting part is the beach is actually artificial, which is 500m long and 1000m wide. The beach is surrounded by a range of hill, covered with a thin forest of pine tree. Near the area is a number of hotels and resort with different type of architecture.
One small tip, if you want a more deserted beach area, is to book a Halong Bay cruise tour to see Titop island – the water is much calmer and much less polluted. Another ideal option is to go 1 hour further to Van Don beach, a hidden gem of Halong Bay caves and bay complex that not many have discovered.

 
2/ Tuan Chau Beach (Ha Long)
Tuan Chau Sea lies on Tuan Chau Island, overlooking Ha Long Bay, the beach here can serve a large scale of swimmer with its coast expanding more than 2km.
Sand at Tuan Chau beach is artificial but on the bright side, the water is clean and calm.

 
3/ Do Son Beach (Hai Phong)
20km to the Southeast of Hai Phong City is Do Son Beach. The place is lucky to possess relatively warm climate in winter but cool in summer that help it to attract customers to come for the whole year.
Do Son Beach is well-known for its smooth sand with coconut tree planted around that give swimmer a real glimpse of a typical tropical beachDo Son Beach is divided into 3 sectors suitable for different needs of visitors: The first one located at the entrance of Do Son Town; the second one crowed with restaurants and hotels while the third one is appropriate for people who enjoy relaxing and peace.

4/ Cat Ba Beach (Hai Phong)
The island is a fascinating combination of boundless sea, primitive forest, river, springs, caves, hills, valley and beautiful beaches with gold smooth sand.
Also, tourist may hide a small boat to travel to Lan Ha Bay, or a chain of wonderful beaches with strange name like Cat Co, Ben Beo, Co Tien; and illusory caves like Trung Trang, Hoa Cuong or Thien Long. Additionally, sea-foods here are the most fresh ones may taste with a relatively low price compare to other sea provinces.

5/ Hai Thinh Beach (Nam Dinh)
Located on Western Nam Dinh Province, Hai Thinh Beaches is a peaceful romantic beach which is a very clean beach with smooth sand and absolutely safe for swimmers.
The beach is always full of cool wind blowing, creating soft layer of steam on air that contributes to its natural romantic scenery. In addition, Hai Thinh Beach is still not very developed so tourist who like resting peacefully, forget about the busy and noisy of normal life or other famous beaches may find this an ideal choice.
The locals here are very friendly who offer tourist the most fresh and clean seafood as well as bets service ones may experience.