Hello, I am a big Space opera/science fiction/gundam/mecha fan and today's pick is Kenran Butou Sai: The Mars Daybreak an underwater mecha anime. YES an underwater mecha anime ;) and pretty good one.

Description:

On a Mars covered in ice and oceans, people have learned to survive by building large floating city-ships in which they can live. And by using submarines to travel beneath the ice and harsh winds that rage overhead, humanity has been able to develop a global community. But during a distant galactic war, Earth looked to Mars as important water resource to support their war efforts. Now that the war is over, Earth still values the wealth of resources that Mars provides, and is reluctant to release their firm control over it.
Continuous oppression breeds contempt, and rebellions occurred. And with each failure, the independence movement learned and grew more organized. During this confusion, pirates appeared to prey on the wealth and abundant resources of Earth's occupying forces. The Earth government decides to organize a special military unit equipped to fight on Mars — to settle and suppress both the independence movement and pirates once and for all.