AAQR: Fox U-42 (free, paid) game

Fox U-42 is a u-boat sim where you need to kill merchant ships and escort ships in WW2 without getting killed. It's bird's-eye-view 2D where you give path to your sub and can choose to shoot, and change speed to high or low.

The initial level is easy enough, but dodging escorts can be a problem (trickier than you think). Try the free version and see how you like it first.

http://www.appbrain.com/app/fox-u-42-free/se.oixx.subfox