I think the graphics on SH3 still stand up pretty well. For example, the graphic story you can find at the link below, which I produced, was done using SH3 (note that some pics have had some tweeks in Photoshop, but they are all basically screenshots from SH3:
http://www.subsim.com/radioroom/showthread.php?t=115045
The Naval Academy aspects of SH games are effectively small practice missions where you get to try out navigation, using the deck gun, the anti-aircraft gun and the torpedoes. In combination with a quick read of the manual, they will teach you most of what you need to know to work the sub's various bits and pieces. One advantage doing these 'academy missions' confers, is that if you complete them successfully, you earn 'renown points' which can be used to equip and upgrade your submarine in a campaign, this emulates the more popular and successful skippers getting the best stuff, as they tended to in real life, and so is worth it. The renown system continues in the campaigns also depending on how successful you are.
What the tutorial missions won't teach you is the tactics of evading depth charge attacks after you've torpedoed stuff, for that there are a few tutorials kicking about on this forum, one or two of which explain how the simulation actually detects stuff, and knowing this will assist you somewhat.
Like most things, you just have to try and see what works for your particular style, for example, some people like attacking at night, others in the day, some will go for long range shots, others like to get in close and hide underneath enemy ships etc. Sussing out what works best for you is all part of the fun of course, prepare to lose a lot of sleep by the way, like most good simulations and computer games, Silent Hunter is very addictive.
Chock