The most common kinds of pocket monsters in the Pokémon universe are Water-types. Appropriately, Water-type attacks are equally common, however, more than just the aquatic variety can learn these attacks from the abyss. This allows for many strategies that can be used by trainers who need a Water-type attack during times when sending out a Water-type Pokémon would be bad.
High damage with passable accuracy makes Hydro Pump preferable to other Water-type moves if dealing damage is on one's mind. It can be one of the best coverage moves to have on non-Water-types as it can turn the tide in a battle very quickly against common offensive types like Fire and Rock-type foes while dealing neutral damage against the majority of other kinds of adversaries.
10 Lickitung
Lickitung is a bizarre Pokémon for many reasons. Firstly, it does not seem to be based on a real-world creature like many other Pokémon. Instead, it is more similar to the Japanese yokai known as an "Akaname", a creature with one claw on each finger and a huge tongue that it uses to slurp up the filth from bathhouses.
Its monstrous origin, as well as the fact that Normal-type Pokémon are more able to learn elemental moves on average, allows Lickitung to learn Hydro Pump via TR03 in Pokemon Sword & Shield. Players who like using this pink pal can use this move to counter Rock-type foes sent out against them, which are common counters for Normal-types.
9 Hydreigon
Dragon-type Pokémon can also learn a wide variety of moves, particularly ones from elements like fire, water, and electricity. The reasoning for this is likely that dragons and mythical in nature and can harness the energy of the world around them better than other types.
Hydreigon, as a pseudo-legendary Pokémon, has a massive arsenal of attacks at its disposal, including the mighty Hydro Pump. Like many others, however, it can only learn this aquatic assault through TR03. It may seem cool, but placing Hydro Pump on one's Hydreigon will end up mostly as a waste of time since there are few Pokémon likely to go up against a trainer's Hydreigon that are weak to Water-type attacks.
8 Kangaskhan
This proud-parent Pokémon can do many unexpected things to confuse and confound one's adversaries like using many attacks that foes won't see coming. Hydro Pump, learned by TR03, can be a real surprise for any foe switching in and expecting a Normal-type attack.
Pokemon in the "Monster" egg group tend to exhibit strangely diverse features, much like Dragon-types and Normal-types when it comes to learning moves. Kangaskhan's monstrous origin likely references the mysterious powers that lie dormant in many Pokémon from this group who are simply waiting for their potential to be unleashed.
7 Drampa
This placid Pokémon stands out as it is the only dual-type that is both Normal and Dragon-type. It is also in both the Monster and Dragon egg groups, meaning that it has a huge movepool of attacks to choose from. It can learn damage-dealing moves from every type in the game except Bug and Poison, making it arguably the most versatile Special Attack oriented Pokémon.
Understandably, this means that Drampa can also learn Hydro Pump, though, with huge a huge pool of moves to choose from, Hydro Pump should only be used in a slot if a trainer does not have a better strategy for their niche drake.
6 Exploud
There is no ignoring the threat this Pokémon poses with its nearly unblockable sound-based Normal-type attacks that penetrate through an enemy's Substitute and its capacity to even hit Ghost-types with them due to its Scrappy ability. However, this is not even taking into consideration this Monster egg group beast's excellent move pool.
As with many others, Exploud can learn Hydro Pump with TR03 and can use it effectively against the usual types meant to block Normal-type Pokémon from succeeding. With its high Special Attack, it is not a bad choice for this strike from the sea.
5 Avalugg
Ice may just be solid water, however, it encompasses very different kinds of Pokémon. Water-types are meant to be fluid and versatile while Ice-types are much more rigid in their usability due to heavily favoring either offense or defense.
Avalugg is such a heavily dedicated pocket monster, as it specializes in Physical Attack and Defense with little capabilities in terms of Special Attack. It can learn many special moves, like Hydro Pump, though since it neither gets the Same Type Attack Bonus and has an abysmal Special Attack, it would be nearly pointless in an offensive strategy.
4 Tyranitar
There are a few Pokémon that can learn moves that they have no right to be using due to their biological makeup. Rock-types like Tyranitar get crippled when Water-type attacks hit them, so it doesn't really make a great deal of sense for them to be able to produce and launch the same kind of moves.
Although, this has not stopped the developers at Game Freak from letting it happen. Following the trend with the Monster egg group, this extremely tough Pokémon can let out a whole host of elemental attacks, one of which is Hydro Pump. An unorthodox addition to its moveset as this armored Pokémon has much better ways of dealing with those weak to Water-type attacks like Fire, Ground, and Rock-type Pokémon due to its access to Earthquake and other powerful moves with higher accuracy that take advantage of Tyranitar's much better Attack stat.
3 Aggron
Much like its other kaiju-looking counterparts in the franchise, this dweller of the Monster egg group can learn many moves that it rightfully shouldn't even be able to comprehend. Attacks such as Hydro Pump, as well as Thunder and Blizzard, can be taught to one's Aggron with TRs and TMs.
There is little reason to equip an Aggron with Special moves as it cannot make great use of them because of its very low Special Attack stat. Even with the proper EV training and Nature, there's no way that an Aggron focusing on anything besides Physical Attack can be successful at offense.
2 Rhydon
As one of the first Pokémon ever to be conceived, Rhydon gets mentioned a lot outside of its battle capabilities. It is one of the more generic Ground and Rock dual-types that tends to get lumped in with other first-generation creatures of similar description. People often forget how much move pool coverage it has.
Like others found in the Monster egg group, it has a plethora of attacks, including the TR03 gifted Hydro Pump. Being four times weak to Water-type attacks doesn't stop this stony kaiju from unleashing torrents, though, like Aggron, it won't do very much damage with them. Furthermore, Rhydon is likely the one responsible for starting this trend due to it being one of the first Pokémon to be created, and probably the first one in the Monster egg group.
1 Drilbur
There is no way for Drilbur to learn Hydro Pump according to all official sources. No TR, TM, or tutoring can grant it the ability to launch any sort of liquid cannon, however, this species can still be found with Hydro Pump in one of their move slots due to an error.
Between April 28 and May 11, 2020, raids in a Wild Area in Pokemon Shield (though not Pokemon Sword) contained Drilburs that knew Hydro Pump instead of Scratch. This was clearly a mistake as these Pokémon, and Excadrills that evolved from them, cannot be traded or used in online battles as they technically have an illegal move.