Be warned that 688i might accustom you to certain things that are different in SC and DW! For example, the towed array is not the best one to detect enemy subs in 688i, unlike the real item and what SC/DW model.
Get the latest patch with fixes here:
http://www.subsim.com/subsim_files/patches2005.html#688
I played 688 a lot in its time, have great memories of that but IMHO Sub Command with SCXII/SCU beats it by far. Dangerous Waters is of interest for me because of the Kilo and the community for multiplayer, otherwise I would play only Sub Command as I like it better in general.