How to get / set value of a float2 field in a custom editor script?

If you just want to allow a user to modify the value, you can use the generic PropertyField() to automatically use the editor's default representation for whatever field type it happens to be:

EditorGUI.PropertyField(dirRect, waveOrigin, new GUIContent("Point of Origin"));

But if you want to extract the value they set and read / modify it, then it would be useful to give yourself a couple helper methods:

public static class SerializedPropertyExtensions {
    public static Vector2 GetFloat2AsVector(this SerializedProperty property) {
        Vector2 output;
        var p = property.Copy();
        p.Next(true);
        output.x = p.floatValue;
        p.Next(true);
        output.y = p.floatValue;
        return output;
    }

    public static void SetFloat2FromVector(this SerializedProperty property, Vector2 value) {        
        var p = property.Copy();
        p.Next(true);
        p.floatValue = value.x;
        p.Next(true);
        p.floatValue = value.y;        
    }
}

Then you can work with the Vector2Field as you were trying to do:

var v = waveOrigin.GetFloat2AsVector();
v = EditorGUI.Vector2Field(dirRect, "Point of Origin", v);
waveOrigin.SetFloat2FromVector(v);

Don't forget to call serializedObject.ApplyModifiedProperties() to commit the changes the user's made to the object's data.