There are 2 Deepsea Id cards, they're required to get into the moon base and get more knowledge stones.
You get one on Neptune, by going to the right from the 1st warp point, it's on the right side of the start, you'll find a building which has 4 enemies. 2 of which are Blue Starmen, another is a Ghost of Starman, and a energy robot. Beating them will drop a present which has the first Deepsea ID.
The next one is found by going back to the Celine on Chapter 7. You head to where Captain