Halong Bay has the typical diurnal tide system (tide amplitude ranges from 3.5-4m). The salinity is from 31 to 34.5MT in the dry season and lower in the rainy season.
The best time of year to visit Halong Bay is during the spring or fall. Because the bay is situated in the northern part of the country, it can get colder in the fall and winter, and though you won't see extremes such as blizzards, it's still a lousy place to be when it gets cold. This area is one of the most popular destinations for vacations in Vietnam. Though there will be more tourists in the area in the spring and summer, the area is still a good place to go. In some situations, it's actually better that there are more people there. There will be more places to go for food and more entertainment in the area. When the weather is good you can go out in the bay on a junk, or a boat, and take a tour of the area. You can go swimming if you wish and just explore the beauty of the area. However, you can't do all of that in the fall and winter because it gets too cold. When you visit the area you should pack a swim suit, some shorts and a t-shirt. You should have enough clothes to keep you comfortable.