there is no underwater galactic hideout, the closest hideout you would get to underwater is the aqua/magma base in ruby and sapphire
the team galactic boss gives it to you at the team galactic hideout!
Veilstone City
The boss of Team Galactic (Cyrus) gives you the Master Ball after you beat him in the Team Galactic hideout in Veilstone City. This happens after you beat Candice in the Snowpoint City Gym (7th Gym).
Team Aquas underwater hideout is located in Mossdeep. You will need to dive down in the water and on the right hand side there will be a cave, which will be the hideout.
Veilstone City :) x
You may not join Team Galactic in Pokemon Pearl or Diamond.
I believe you can can find an UpGrade(used for evolving Porygon into Porygon2) in the Eterna City Hideout.
You will need Galactic key and this how to get Galactic key you will need to talk to a Galactic team mate by the Galactic HQ.
veilstone team galactic base veilstone team galactic base
Team Galactic boss is in M.T Corenet near snowpoint city.
Both Pokemon Diamond and Pearl have Team Galactic as the main antagonist team.
at the stone pilar