Can I freeze a tapped creature?
This is legal because Sharktocrab is applying two unrelated effects to the same creature.

Sharktocrab reads:


  Whenever one or more +1/+1 counters are put on Sharktocrab, tap target creature an opponent controls. That creature doesn't untap during its controller's next untap step.


The first thing to note is the targeting restrictions. "Target creature an opponent controls" means that any creature your opponent controls can be targeted, regardless of its other characteristics.

Then you do two things to the target creature: you tap it, and you prevent it from untapping.  But the "freeze" effect isn't dependent on the tap effect.  They are just both applied.  In the case of the tap effect, it will have no effect on already tapped creatures.  But the freeze effect gets applied anyways.