another honorable mention, one I've been turned on to and trying out lately, is the Guerrilla Submarines Spyron Mod, which has a different flavour than RSRDC, they can't overlap, and it changes the campaign structure of TMO too.
But, looking for something new on my RFB install, I decided to load it up in place of RSRDC and was greeted with my first mission on December 1941 in USS Gar being a supply drop at Corregidor. The usual "sink ships" and "patrol area" missions are there but I'm very interested to see what else I can do.
http://www.subsowespac.org/silent-hunter-4-mods.shtml
EDIT: I do notice SH4 having a very niche club of people who still play it. I can't say I'm a regular player, I haven't touched it for a good couple years before last week. Oh, the lovely side effects of ADHD. Sometimes its nice to kick back, read a book, and listen to the news from CBS amongst the Captain Midnight serials enroute to your patrol area