We were surprised that there was a charge on the Mercury-as the Constellation didn't charge us. Our last cruise was on the Mercury and if you buy the package while in port, on the first day-you get a special reduced rate. The Thalassotherapy Pool is much smaller on Mercury, but more quiet and peaceful to rest. I believe we paid $80 each for the 12 days were were gone, and the regular rate was $20 each, per day-so it's quite a savings. Love the pool-well worth the money!!