Hi,
X axis is left and right across the sceen, Y axis is up and down, Z axis is in and out of the screen.
I am assuming you are rotating your watermark so that it turns around the Z axis, which would be like a windmill viewed from the front, Y axis rotation would be like helicopter blades, X axis like a paddle steamer viewed from the rear!
so much for steam age engines...and fcpx has steam age animation capabilities...so can only do Z axis as far as i have discovered. Motion can do more and might be worth your investigating i think...but it doesnt do much that i need,
as i said i use Maya which is certainly a full blown animation, modelling and VR creation app, it can render photoreal movies of any resolution , fabulous lighting synthesis...blahblah...i could go on for hours...put aside a couple of decades if you are thinking of getting involved.
Your animation if I am reading it correctly should be easily achieved by keyframing in the inspector, try keys at 0 ,180 and 359 degrees.
Then if you copy and paste the clip you will end up with a looping result.
If you want to combine a translation animation with the rotation then it might be best to get the rotation working well and then make the clips into a compund clip and translate that...speaking here from my imagination as i am not in front of fcpx...so if it doesnt work you will have to call it an experiment!
I will try it next time i am in front of my machine and see what other issues i find.
have fun
geoff