If like a fish doesn't need to be be a fish, almost every edition of D&D had sea elves or aquatic elves:
Aquatic elves are water-breathing cousins to land-dwelling elves. They
live amid the waves and the ocean depths with allies such as dolphins
While this may be a little bit cheated in terms of what the hobby can do, because basically the rulebooks take land elves, let them breathe underwater and have dolphins instead of horses and cows, it still is a scenario you described.
Chances are good you already own some edition of D&D, so looking them up in the monster manual and asking your girlfriend if that's something she can have fun with should come at no additional cost. The monster manual will give you some basic ideas about common allies like merfolk and enemies, like sharks and sahuagin.
Edit: some more details as requested:
Not every edition of D&D is suited for this style of play. I have played AD&D, AD&D 2e, 3.5e and they seem very suited to playing survival adventures. From what I have read, basic D&D from the 70s, 3.0 as well as D&D Next seem to be suited as well, although that's a theoretical assumption based on the rulebook. I have played 4e and I will agree that this is not a good fit because it does not provide the framework for survival challenges. It's more of a combat-centric boardgame with a story attached. However, we have stopped playing 4e when this became clear, so there may be extensions or additional rulebooks providing such a framework that I missed.
I have played countless adventures where survival was the key challenge. Most of them had either a desert or an ocean theme. Both work fine. The monster compendiums have tons of predators for any climate that are scary on it's own, even without problematic resource levels. There are rules about how much food you consume in a day, how far you can travel in a day and it's simple maths to craft an adventure from that.
I have been in groups where players had sea elf characters and they work just like normal elves for all rules except breathing.
From a very basic, pattern matching point of view, there is no difference between protecting a villages herd of sheep from wolves or protecting an underwater villages herd of baby dolphins from a shark. Same rules, different paint job.
On a personal level, you should look into what your GF wants from the adventure. The challenge sounds like it should revolve around social interaction and interaction with the environment. Solving problems, escaping danger and building relationships. Those subsystems are quite simple, but effective in the mentioned editions of D&D. The fact that most groups use D&D as a simple hack&slash game for mindless fun does not mean you have to do so, too. Be creative and use all the rules, not just the combat subsystem.