Your question suggests that you may be confusing the Equivalence Principle with the Principle of Relativity. However, following your title, I have provided an answer regarding the Equivalence Principle only.
The Equivalence Principle tells you that local observations cannot distinguish between acceleration or a uniform gravitational field. The locality and uniformity restrictions are key (and, in fact, complementary) A gravitational wave is, but its nature, not a uniform gravitation field; it has a spatial scale (the wavelength) over which its distortion of spacetime varies.
When a gravitational field is nonuniform, it can easily be distinguished from an accelerational effect by making measurements over an extended region. However, such observations are, of course, not local. The locality criterion essentially means that you cannot distinguish gravitation from a fictitious translational force unless you compare measurement that are made over a region of space that is comparable to the scale on which the gravitational field varies. For a gravitational wave, this means that to observe its wave structure, you need to make measurements over a region that is at least comparable in size to the wavelength.