The Johto region brought several new Pokemon to the series, ranging from powerful to pretty underwhelming.
Water-types like Feraligatr and Suicune are incredible creatures. Feraligatr is tenacious and Suicune is majestic. Not every Water-type Pokemon in Johto was lucky enough to be those two though.
Out of the several new Pokemon added to the Pokedex, there were bound to be a few that were on the underwhelming side. It is a pretty well-known fact that not every creature in the franchise can be the strongest.
Top 3 most underwhelming Water Pokemon from Johto
#3 - Octillery
Octillery really has one of the coolest names in Pokemon. It wasn't give the chance to live up to that name, however. It has pretty good Attack and Special Attack, allowing it to use a diverse set of moves.
But a pure Water-type with low Speed is always going to struggle against the very common Grass and Electric-types. The HP and defensive stats aren't good enough for Octillery to withstand multiple hits before it can dish out multiple hits.
#2 - Corsola
Corsola is absolutely incredible in its Galarian form. When it first appeared in Johto, however, Corsola was nothing special. The concept and design is neat, but much like Octillery with its name, Corsola doesn't do much other than that.
It has awful Attack stats, HP, and Speed. Its Defense and Special Defense are okay with a base of 85 each. That's it, though. Corsola is another pure Water-type Pokemon that can be dispatched quite easily.
#1 - Azumarill
Prior to the introduction of Fairy-types, Azumarill was yet another pure Water-type Pokemon. It has solid HP with a base of 100, but does nothing to stand out. It eventually received a Baby Pokemon at the start of the evolutionary line too.
This made Azumarill a third evolution. As the third and final evolution in a family, it should be way more capable. Instead, it was given lackluster Attack and Defense, along with abysmal Speed. At least it is immune to Dragon-type attacks now.