You should finalize your paperwork at the chartering agency and pay any balance remaining. Be sure to inspect the boat before you set sail, to make sure everything works and all the promised amenities are available.
Things to check include:
Enough life vests for everyone on board
A full gas tank
A full water tank
Sails in working order
Working and up to date radio equipment
An anchor in good condition
A working motor
Sufficient battery levels
Working refrigeration
Working stoves
Plumbing
Backup generator
Tools for minor repairs
A first aid kit
Check to make sure your provisions arrived on time, if you ordered any. Some companies offer a "starter's kit," a package of consumables such as toilet paper, paper towels, liquid dish soap, shampoo, etc, but you should prepare your own in case these run out. Finally, check to see if all members of your crew are present.