There is no official support anymore from SCS, but the game is kept alive by some really really cool modders. There are different mods available, Lwami was the first, it is still being updated and is very challenging. It corrects most bugs of DW and imrpoves the database, as well as doctrines, ai behaviour etc... Also adds a lot of new units in the database. The only downside is that is doesn't add new playables. This is where Alfa Tau 3 and RA keep the stone rolling. Alfa Tau is based on Lwami but adds a whole lot of new playables (italian subs and frigates, russian subs, helos, airplanes etc...). Very well done and available on subguru's website. The other mod RA is being done by the russians and goes beyond what is done in Alfa Tau. Some units such as the frigate are vastly improved (giving you very precise control on the helos for example). This mod is still in beta, so its not publicly available.
People still play online. Thing is there is no dedicated server, so the players must agree beforehand when to play. Also nobody can enter a game already running. Thats why playing in a virtual fleet is the number one option, you can also find freelance players on cadc (another excellent website dedicated to dw, besides subguru and of course subsim).
You can learn more easily and rapidly when playing in multiplayer with other experienced players. Some will even teach you the different units. It makes the entry barrier so to speak much easier on the newbie.
If you like multiplayer, wait till you try multiplayer-multistation that is several people manning a same unit. Think big, a scenario where you have 10 players, 2 on a same sub, 2 on a p-3, 4 on 2 akula II's and 2 on a frigate with asw helos. There is a lot of fun coming your way. The really cool thing is DW doesn't use a lot of bandwith so even with small upload speeds you can host games for 8-12 players, sometimes even more.