When Moby Dick died, the experience bar of many party members was filled and they finally got to level 8.
They got a lot of experience while challenging the different modes of the Underwater Tunnel Dungeon, however, they couldn't be compared to the experience gained on the Hell Mode.
A proof of this is the experience given by the bosses. On Normal Mode, the bosses give 1.000 experience points, which when shared by ten people, each one receives 100 points.
The experience given by a boss on Hell Mode is five times the amount on Normal Mode, plus a 10% bonus for it being the highest level of difficulty of the Dungeon. Only 10-man dungeons and above gives the extra experience on Hell Mode.