Good question

She's the most op character since she's based off all X's armors lol/j

Jokes aside, It's pretty hard to find the answer, especially since there's no armor was specifically built for swimming purposes.
After all, when you're designing an armor you gotta make it efficient and practical.

You could say that all armors function the same, except for X5 and X6 cases where doctor Light made Falcon Armour with lighter metal so it's better suited for flying, while Gaea Armour is built for heavy defence.
I haven't played X6 for a while but I remember Blade armour is just a worse version of Shadow armour.

Long story short: she's not based on any armour since all armors are built to be practical in all environments.