The British National Maritime Museum at Greenwich certainly is one of the most important and largest maritime museums in the world. It holds a large collection of maritime art, British and Dutch paintings, cartography, navigational instruments and ship models.
There is more information about the museum and on-going events on the homepage of the Royal Museums Greenwich.
I was most impressed by the collection of paintings they displayed. It was a joy and very interesting to see these famous paintings. The original works revealed much more details than I remembered from the prints I had seen before in several books.
You can browse the fine art collections of the National Maritime Museum and buy large format prints of the paintings online. They show the prints by 600 * 350 pixels which is quite good for selecting.
In the museum there is also a small but fine selection of contemporary ship models displayed. However, there is a photography ban, and they enforce it. When I took some pictures of their Bellona class 74 somebody came and asked me to stop it. For anyone seriously interested in details of rigging and hull that is a real nuisance. (I forgot to ask him whether making sketches is allowed).
I perfectly understand that photography is forbidden with the paintings as they get damaged by people using flashes when they believe to be unobserved. And there are many prints of the paintings available. However, on wooden ship models the photography ban is something the museum management should reconsider (status April 2009).
So, if you are interested in details of naval history and want to memorize them by taking pictures you better visit the Science Museum, London, the Musée de la Marine, Paris, or the Deutsche Museum in Munich (taking pictures is allowed, but no flash, no tripod).