Beside at least with Horsea line it was showed how they are able to battle on dry land to some extent, like Kingdra for example.
Personally i welcomed that Misty had fish based pokemon too, making sense for water trainer to collect aquatic pokemon and it made her stand out compared to others. I only wish writers utilized them more, and if Misty returns hopefully we might see Kingdra with her.
Agreed. Its pretty clear how Staryu and Corsola were among Misty strongest and with most battling experience pokemon, with Staryu showing to be resistant and well trained being of great help. One of his most memorable moments was in third movie when being on pair with Molly Mantine which was double stronger than normally he would be thanks to Unown. And Corsola had excellent defense abilities catching opponents of guard with victories in Whirl Cup, against Dorian and later in hosos being confirmation to unique abilities this pokemon has.