When you're lucky - that means, when there is a bit less water as usual - you can see a very old shipwreck appearing on Kastellholmen. Don't expect anything like the Vasa, but you can clearly see that it was a ship. "Grå Ulven" (Grey wolf) it is, a danish warship that sank in 1642. It was only discovered in 2013 when it was visible for the first time. As Kastellholmen is worth a visit anyway, have a look when you're there, maybe you can see it.
If you're interested in ships, there is of course the famous Vasa-museum but also the very interesting Vrak-museum. Both worth a visit, even when you're not a ship-entusiast in general.