First thing's first, make sure your gear is up to the job. Unless you are fishing with one of the big game Penn fixed spools I would take a strong multiplier. A 5lb tuna will put a 20lb salmon to shame, and having fished from a rig off the Ivory coast myself I know that there are plenty bigger than that. I wouldn't go any lower than 35lb line and make sure you take plenty of it as well as plenty of big stron single hooks, you can go through an awful lot of gear in an awful short time. I am currently working off of India and although we are not officially allowed to fish I still use a handline and it is nothing for a tuna of anything over 30lb to smash 200lb breaking strain line if it decides it wants to run. Fantastic fun, but it can be hard on the hands, even wearing leather gloves. As to lures, I wouldn't bother with anything fancy, or expensive. The most succesful lure I used over there was a piece of stainless piping around 5" long then about one foot trailing behind it a piece of 1/4" purge tubing a couple of inches long covered in reflective tape with your hook set in the back. Any longer than a foot behind the steel tubing and it became difficult to cast. The method was to cast as far as you could and wind back as fast as possible so that the steel tube and trailing tubing would skip across the surface. You cannot wind too fast for tuna.... There should be a good few dorado down there too, they are just as exciting but you lose an awful lot more, they are much better eating than tuna too. If you are getting the galley to prepare the tuna, make sure they cut out all of the blood line down the sides as this really spoils the taste.
Down in India there are a lot of flying fish which can get the tuna fired right up. The lure I use for them is just a 5" long piece of broom handle, roughly shaped and covered in reflective tape with a hole bored through the centre for your hook. Again this is stripped back as fast as possible to skip across the surface. If a trout sipping down your dry fly sets your pulse racing, try watching a 50lb tuna rocketing up and smashing into your bait and continuing 6' out into the air!
I am currently on a drill ship, so just haul the fish the few feet over the side, but off of a rig you will need to get the welder to make up a sliding gaff that you can send down the line on a rope to get your fish up.
Tight lines and have a good trip.
