The important thing is whether the magical acceleration is relative to herself or relative to a rest frame.
In the second case, she can encase herself in a "water bubble" and accelerate the bubble however she wants. If the water needn't be in contact with a larger body of water, then she can shape a large freesbee-shaped flying saucer and zip around on it. This sounds a bit too easy to me, though.
In the first case, this won't work, but we can use three different methods.
One: she submerges, and magically accelerates the water in front of her so that it parts, going backward, at the same time forcing the water all around to stay still. This creates a suction that will move her forward. New water will flow in the acceleration zone, and be expelled backwards.
She is essentially "paddling" with a large tube-shaped invisible magic oar.
If she can continuously shift the magic from the water being expelled backwards to the inrushing water, and keep the outer water "funnel" stable, then she can create a magical hydroturbine with a maximum theoretical speed of Mach 4.4.
Two: she accelerates several pencil-thin jets of water inward and in front of her. The water doesn't give her momentum, but it immediately mixes with the remaining water all around, creating a current. She can swim in the current.
Three: she creates a magical water torus with herself in the center, with the water rushing in from the direction she wants to go. The water in the torus rotates like this
This magic "smoke ring" in the water has a turbulent surface on the inner border. This will force other, normal, non-magicked water to be sucked in and expelled, as if from an impeller, which again will allow her to flow forward at a significant speed (bringing the magical torus along with her, and experiencing no drag from it because magic - the same lack of action/reaction that prevents her from exploiting a water jet now plays in favour).
This is possibly easier to do, concentration-wise, because the impeller can be made of the same water continuously rotating (there is no need to continuously re-magick new water). It is not too different from creating a pair of invisible propellers out of magicked water and having them rotate.
The whole volume in which the torus and the swimmer are immersed can be imagined as a "physically consistent and law-abiding black box", wherein water is sucked from the front and ejected at speed from behind. Therefore, the whole volume is propelled forward.
A similar effect, requiring more control, would be to create a zero-acceleration ovoidal shell (so, a shell the water cannot go through - made of water and magick instead of metal, but still a submarine shell). At the front and at the back there will be two "holes" in the shell, and water would be accelerated inside the first and outside the other, creating a MHD (MagickalHydroDynamic) drive setup.