Dangerous Waters is a strict upgrade, in my opinion. 688(I) H/K is rather old at this point and certain functionality you take for granted in DW is absent from 688(I), like the use of mouse wheel scrolling on maps is absent in 688(I). It does have some interesting features that DW does not have (plotting a history of the depth beneath the keel, panning around the control room), but nowhere near enough for me to recommend a game two generations older than DW in an either/or scenario. Sub Command, the sequel to 688(I) and prequel to DW, has some fun Cold War-type campaigns that are all purely sub based, which you can play in Dangerous Waters.
There are also a lot of good third-party scenarios for DW on this website here:
Dangerous Waters Custom Scenarios