playing Germany i always start with building a navy with a variety of ships. I do this for two reasons: 1: it sets the allies invasion of Europe back a few turns which gives you a better chance at Russia. With control of the atlantic you can free some of the troops guarding Europe to push harder at Russia. 2: It allows you to hit convoys.
While building the navy you have to be careful however, if you try to hold control of the Atlantic too long and don’t shift your focus to Russia then you run the risk of being invaded by Russia. The navy is merely to delay the inevitable invasion of Europe. I have used this many times and if done right, I have played a total victory game against experienced players, and beat the Allies. If done right, and your timing is excellent, then not even America can save the Allies, I have even successfully invaded the United States. (yes it was a long, difficult war)
The German navy works great, but it has to be balanced with a timely shift to land forces and a taking of Moscow.