Revell RC U-boat

Also try The Sub Committee (http://subcommittee.com/)

They are a great all-around sub resource, but heavy into RC with a forum and heaps of resoruces. 

A couple of leads:

1.  Lots of good info on the subcommittee website.

2.  Dave Welch makes water Tight Cylinders for use in various plastic sub kits, including the Revell Type VII and  the Gato class.   Check out his website at  http://www.frontiernet.net/~bwelch/

 Hope this helps...