It's been over a month since the last reply, so new prompt.

Character: Mash Kyrielight/Mashu
Source: Fate/Grand Order(FGO)
Setting: Gudako and Mash are on a mission to defeat a Hydra after it appeared at a beach. Things did not go well for them(though Gudako escapes mostly unharmed).
Wants: Drowning, underwater sex, hard vore, guro(optional)