To get to Water Ruin, defeat Mizar 1 with Lupus. The characters' armor 
will be upgraded. Then, go to Sekmet with any character. Go to the third 
room and get on a Jetpack pad. The Jetpack meter will fill up about 50%. 
Fly to the high catwalk. Walk through all the rooms until you find the 
Launchpad. Take off in the ship and it will land at Water Ruin. Then 
dive as Vela into the cave under the largest island. The nitrogen tank 
and some weapon upgrades will be on the table.