650/X-2 Kawa 650sx Question

So my buddy and I were on the lake today and he started to have problems with his ski. This is what is going on:

-Hit the gas and the motor rev's but SLOWLY moves.

-Once it hits a point it goes full throttle and acts like nothing is wrong.

I was thinking maybe spun bearing? Impeller is loose? What do you all think?
 
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