I can tell you right now that lapras, feebas, and seel may be a bit difficult. While Hoenn is indeed a heavily aquatic region, much of the gym circuit takes you inland—up a volcano, no less. Even if your boyfriend’s sister plans on keeping her starter in its poké ball for much of her journey, the dryness of Mt. Chimney, Lavaridge, and the Hoennian desert may make it difficult for training pokémon not well-adapted to land.

Of the remaining three, dratini may be the most difficult to train. It doesn’t learn powerful moves right off the bat, and it’s slower to raise and evolve than the other two, even considering the fact that one of them can only evolve via exposure to a water stone. However, it is the most powerful option in the long-run, so it really comes down to how much patience your boyfriend’s sister is willing to exercise.

Meanwhile, staryu and marill (or azurill, depending on whether or not you plan on giving your azumarill sea incense before breeding it) are both perfectly fine choices. Both are fully capable of traveling on land, both are equally easy to raise, and both learn moves rather quickly (compared to dratini). Staryu may be a little more challenging to bond with because it’s not as expressive as marill, nor is it quite as affectionate, but on the other hand, marill’s secondary fairy typing instills in it a natural mischievousness that may be a challenge to a trainer who’s not quite expecting it. Additionally, marill may be easier to raise (and certainly easier to evolve), in part due to its ability to bond with its trainer and in part because its defensive capabilities make it more difficult to defeat, but on the other hand, staryu and starmie learn a wider variety of moves and have generally better offensive capabilities.

In the end, I would recommend either a marill or staryu, as those two may be the easiest for your boyfriend’s sister to train. Between the two of them, you would have to ask yourself if she would prefer a more defensive, more affectionate pokémon or a mysterious pokémon that’s a bit faster and can hit harder by default.

Of course, as always, be sure to ask the new trainer what she would like as well. She may even be willing to train a dratini if she has her heart set on something a bit more mystical.

But … try to steer her away from the idea of raising a seel, feebas, or lapras in the heart of the Hoennian mainland. Just … just trust me on that.