Although a sequel to 2010's "Piranha 3D" seems entirely superfluous, don't tell that to the target demo of gorehounds and softcore-deprived teens (if they still exist) who may still take some notice. Regardless, the sub-par franchise installment "Piranha 3DD" will quickly sink beneath the waves to find a suitable resting place in video, where it will surely tempt younger viewers to outwit the R-rated restrictions.