A wonderful museum! I've heard that some people come to Hamburg specifically to visit this museum! I believe it's worth it. In short, it contains personal collections of different collectors, and they have only one thing in common - the theme of the sea and the conquest of the water element.
When you plan a visit, keep in mind that 3 hours will not be enough. On the top floor there is a wonderful collection of paintings about the sea and ships. If you are interested, start with it and go down. Otherwise, you simply won't have enough strength.