New costumes
     After  beating the game, start a new game + on hard mode, and you 
will find a treasure chest on the Sunny with the key to lockers. Go to 
girl's  room,  and  you  will  be  able to change the costumes of your 
characters



Unlock special combined attack
     Finish  all  50 rounds of survival mode, and a chest containing 
a certificate will appear. You can now perform a new special attack with 
Nami, Usopp, Robin and Brook



Unlockable Characters/Modes
     Beat  certain  bosses  on  the right difficulty to unlock them as 
playable characters in versus mode.

Aokiji:        Beat him him on normal difficulty

Arlong:        Defeat him in Survival mode

Calgara:       Defeat him in Survival mode (20th round)

Enel:          Beat him him on normal difficulty

Evil Guardian: Defeat it in Survival mode

Final Boss:    Beat him in Hard mode

Hard Mode:     Beat the final boss on island 5

Krieg:         Beat him him on normal difficulty

Kuma:          Beat him in hard mode

Kuro:          Beat him him on normal difficulty

Monster 
Point Chopper: Defeat him in Survival mode

Moria:         Beat him him on normal difficulty

New game + (and transferable save to Unlimited Cruise 2): 
               Beat the final boss on island 5

Nightmare 
Luffy:         Defeat him in Survival mode

Spandam:       Beat him him on normal difficulty

Survival mode: Read monument on final island

Wapul:         Beat him him on normal difficulty