Wikia

The Hydro-Pack was a black market upgrade to Clank that you could acquire in the first Ratchet & Clank game on Planet Hoven. It could be bought from Edwina's Robo Shack for 2,000 bolts. The Hydro-Pack was a useful upgrade that allowed Clank to use jets to propel him and Ratchet underwater much more effectively than if Ratchet were to swim on his own. This feature also allowed Ratchet to swim against currents to get to secret areas, like up the river in Tobruk Crater, Novalis (which leads to a gold bolt), and navigate water storages that have a timed electric current running through the water. This upgrade didn't allow Ratchet to avoid the large fish that dwell in many waters.

Clank kept the upgrade for the later games, and so it was immediately

available from the beginning of the game.

On the European Going Commando site, it stated that the Hydro-Pack was fitted with Torpedo Launchers, but this feature never made it into the final game, so Ratchet was still vulnerable to any creatures that may prey on him in the water.

In Ratchet & Clank Future: Tools of Destruction, the Hydro-Pack can be equipped and used like the Thruster-Pack.

TriviaEdit

You can equip the Hydro-Pack in Ratchet & Clank outside of water but it will do nothing and will be the same as Ratchet without Clank.
The Hydro-Pack was claimed to be a Gadgetron gadget. However, Gadgetron's manufacturers Minton Inc., were involved in on-going licencing issues, which caused Gadgetron unable to stock the gadget. Gadgetron's lawyers Brushling, Flynn and Culverhouse were mentioned to be working on the issue.
In Tools of Destruction, the device was still controlled with the R1 button, and was the only R1 feature that kept its place, whereas all the other R1 functions were switched to R2.
Oddly enough, the effect of the Hydro-Pack still worked for Ratchet in Ratchet & Clank Future: Quest for Booty, despite him not having Clank on his back.
In Going Commando the Hydro-Pack was supposed to be first obtained on the alternative path in Canal City however was replaced with a nanotech upgrade.^[1]
A modification was planned for the Hydro-Pack in Going Commando called the Gunsled, which would have been used for underwater gameplay, but was cut from the final version.