The main point of making this type was to split up the most common type: Water (133 out of 802, in other words, every 6th PKMN is a Water type), to add diversity and give all those underused fishies some spotlight.

It sounds super silly and redundant at first, but hear me out.

Ever since the Tynamo-line (along with Stunfisk) got introduced, I've realized that being an aquatic creature does not have to equal being a Water type, and I have hoped that we'd see more and more examples of non Water type fish and other aquatic creatures (Malamar, Dragalge and some fossils come to mind). The Tynamo-line is also the perfect example because it doesn't learn practically any Water moves.

Similarly, the opposite holds true as well. Being a Water-type does not have to equal being an aquatic creature. Meaning you can have Water PKMN that has Water slapped onto them as an element, with some prime examples being Simipour, Suicune, and Keldeo.

So Aquatic as a type would on the defensive side be akin to some of the "species" types (Bug, Fairy, Ghost, Dragon..), in that sense encompassing most fish-based PKMN and sea-dwelling creatures, while on the offensive side it would be like a "skill" type (Flying, Fighting, Psy, Poison..), which would include being able to battle well in water, swimming up Waterfalls, Diving, or using body-parts more common in certain aquatic creatures (Crab hammer).