Hamburg has the Cap san diego which is a great cargo ship and she still occasionally puts to sea about a mile up the road is the U434 a Russian Tango class submarine (you can do both in less than half a day)
https://www.flickr.com/photos/131313...57666733027292 (Cap San Diego)
https://www.flickr.com/photos/131313...57666339267990 (U434)
The type XXI in Bremerhaven you only need around 1 hour with it unless your going to view the entire museum and go on every single vessel in that area (some charge individually) but it is well worth it.
https://www.flickr.com/photos/131313...57666343430700 (bremerhavn museum)
Just across the way is Wilhelshavn (1 hour drive) home to Molders and the U10 & some other bits and is again worth a trip
https://www.flickr.com/photos/131313...57666770682041 (wilhelshavn museum)
U995 is probably the pinnacle of your visit and the memorial is worth the visit too (it is across the road the big tower) spend some time here for certain this not just with the submarine but the memorial and also go into the tower not only go to the top but also on the ground floor you will see a ramp behind you as you walk in make sure you go down it and look below there's another memorial underground, also be sure to visit the models you will pop up in a separate location to where you went in
If you get time U11 a Type 205 is in Fremahn Island she is almost identical to the one in Wilhelmshavn and theres a life boat to see from Hamburg your talking 2 hours driving
It would be nice to see your pictures up here too and how you felt around U995 especially so please be sure to share it with us, these submarines hold a lot of history and every angle every picture every emotion should be captured from every person.
Good luck and if you require advice don't hesitate to PM
Good luck and Enjoy