Raticate: Aquatic Pokemon?
One of the Pokemon with the most aquatic Pokedex entries is not a Water type, and it essentially learns no Water-type moves.
The First River Result
When you think of Pokemon that live around rivers, you probably picture Slowpoke, Marill, maybe Magikarp. But the first Pokemon that appears when you search the entire Pokedex for "river" is... Raticate?
Multiple Pokedex entries talk about how it has webbed feet, swims, and even hunts for prey in the water.
"Its hind feet are webbed. They act as flippers, so it can swim in rivers and hunt for prey."
"The webs on its hind legs enable it to cross rivers. It searches wide areas for food."
"Its rear feet have three toes each. They are webbed, enabling it to swim across rivers."
"People say that it fled from its enemies by using its small webbed hind feet to swim from island to island in Alola."
Swimming in a river is one thing. Hunting prey in water is impressive. However, Pokemon Sun takes it even farther.
"Its hind feet are webbed, so it's a strong swimmer. It can cross rivers and sometimes even oceans."
OCEANS. Not just streams or rivers. OCEANS.
This is well beyond the capabilities of something like a normal rat, and more like what you would expect from a marine mammal. That is why some fans speculate that Raticate's design may be based partly on a muskrat: its body is more compact than a typical rat, and its official lore keeps returning to water.
The Learnset Gap
Raticate is not just an overgrown sewer rat. It is apparently a semi-aquatic, ocean-crossing predator. So why can it not learn Surf, or basically any Water move for that matter? You can check Raticate's learnset against its entries and the mismatch stands out.
It learns Water Gun and Bubble Beam via TM in Gen I, but that can hardly be said to fit such an aquatic lifestyle.
Size Check
Perhaps it is too small to learn Surf? Certainly crossing an ocean on ones back would be a harrowing adventure. Raticate is listed at 2'4" tall. That is not huge, but it is still more than twice the listed height of Chewtle and Froakie, two much smaller Pokemon that can be used for surfing in the modern games.
Loading Raticate size chart...
How Game Freak Could Lean In
A Water/Normal typing would be one direct way to acknowledge the aquatic lifestyle, though it might feel a little like a different version of Bibarel. Another idea is a signature ability, maybe called River Rat, that makes Raticate's Normal-type moves super effective against Water-type Pokemon.
Raticate is not a fan favorite, and probably is not going to get very much love any time soon. It did get an Alolan form, so it may be waiting a while for another spotlight.