Contrary to popular belief, it’s not true—many examples of both types can swim. This is especially not true for steel-types, which consist of pokémon that have the flexibility to physically swim or that have abilities that help them repel off the planet’s own gravitational field enough to keep them afloat. (That last one is not actually a pun. I do mean they literally bob in the water and fail to sink solely because their magnetic forces keep them from doing so.)
That having been said, at the risk of putting things bluntly, your mawile has a fear of water because you taught her to have a fear of it. This isn’t me accusing you of wrongdoing; your fear was indeed valid due to how much that rumor has been perpetuated in our way of thinking about steel-types. However, it is important to know because it’s the key to helping her overcome her fear.
What I’m trying to say is you need to teach her that it’s safe. If you can, go back to the same pond. If you can’t, any large body of water will do. Once you get there, get in the water yourself. Swim out a bit and show your mawile that you’re fine. She doesn’t need to get in herself at this point; she just needs to see you work with water so she understands that it won’t necessarily hurt.
Then, start with a bit of exposure therapy. Begin with smaller bodies of water: a bathtub or a very slow, very shallow stream will do. Have your mawile rest in it for as long as she feels comfortable, then take her out, dry her off, and give her a treat. Repeat this until she grows used to the smaller body of water, then move up to the larger. Once you get to sufficiently deep bodies, you may also wish to give her a few swimming lessons. (Some towns have water-type specialists who offer swim lessons for non-water-types, but if this isn’t an option due to finances or the lack of one yourself, just know that swimming for a mawile is very similar to swimming for a human. The only difference is that they need to exert more effort in keeping their upper bodies above water due to their false jaws.)
Either way, work your way up until you can return to the pond and let her swim. Take it easy and maintain a good hold on her until she’s relaxed and capable of either standing at the shallow end or swimming on her own.
It will take some time, yes, but with effort, consistent rewards, and a lot of patience, you’ll be able to teach her that there’s nothing to worry about when it comes to water.
Best of luck!