My dad was in the air force, so I was there for two years as a kid, my mom is from just outside of tokyo. There is a bit of harsh feelings there lately towards the marines that are reckless at night as they don't face the japanese criminal system. But overall there are no problems with being an american over there. Take the time to learn some basic japanese as well.
The country overall is an amazing mix of technology and tradition. Make sure to take some time to get out of okinawa. Check out some of the old temples and old towns. They are absolutely gorgeous, the transit system will take you anywhere you want to go. Of course visit tokyo, there is so much to do there I can't even begin to cover it. As far as the marine aquarium hobby goes there are lots of stores in the tokyo and yokohoma area, I don't know about the other areas. Make sure to visit the aquariums there as well. There is a sea park in yokohoma with a three story circular tank with tuna, a wonderful penguin exhibit as welll as some cool reef tanks. Then there is the other aquarium that I haven't visited with the whale shark.
Lots lots more, but I gotta go to work now...