The Dry Dock is a place where you can buy stuff from the Merchant, customize your ship's equipment/switch ships, and bit on Elite items at the Marina.
Here you can customize the equipment on your ship, take stuff off, put stuff on, switch ships. You can interchange your equipment between your subs, but there is a waiting time after your unequip an item, depending on how much stuff you take off of one ship.
Here you can buy items such as new subs, cannons, hull armor plating, and ammunition. The non-eite items are bought with Cel, which is common currency, while the Elite items are bought with rare Helix.
Here, you can bid on elite items for cel, including, but not limited to, an elite ship, elite ammunition, elite torpedoes etc. It is a blind auction, so you can't see what the highest bid is. Also, if you bid and aren't the highest bidder, get outbid, etc, your cel won't be reimbursed, so bid carefully!