Go to the left and open the chest to get an Ether, and the All materia if you don’t have Yuffie, but I think if you do have her, you can get to the materia, you might be able to get it if you answer the bottom option. Anyway, talk to everyone here, especially Red XIII on the deck :). A guy in white on the deck, not the one running around will sell you Potions and Phoenix Downs and some rejuvenating drink. There is a Save Point near Red. Go back down below and talk to Aeris again. You can now sneak to the front of the ship and check out Barret in his sailor suit :)!!.
Talk to him a few times and the alarms will sound. Go back to the main deck area, and you will talk to your friends for a bit, then choose you party. Equip Quake, Fire and Ice magic, as well as Shiva and any other good materia you have. Save at the Save Point in the northern part of the deck. Go down below and you can now pick up the All materia that Yuffie was blocking before. Go into the door to the north and make your way up the ladder to the left and go across the walkway to get a Wind Slash from the chest. Go back down the ladder and talk to the officer in red. You’ll find out he’s dead, and Sephiroth will appear and summon a version of Jenova for you to fight….
Primarily use Quake, Fire and Ice magic, as well as Limit Breaks. This boss has some nasty attacks such as Tail Laser, and W-Laser which hits twice (obviously!). Keep your HP up, and only use physical attacks if you are low on MP. Jenova can cast Stop on a character so don’t rely on a character too much. You’ll get a White Cape when you win.
After the battle you party will talk, and Cloud will try to explain the situation. Your party will leave and the ship will dock at Costa de Sol.