You can have a look-see through some recent threads and in the Sticky for specific brand choices on strobes, but here's a list of what you will need to help you sort through some of the confusion on the extras:
You need the strobe. This is actually just the strobe head where you put the batteries and where the light comes from. These are sold individually or as sets with some/all of the other bits below.
You need an arm for each strobe. This attaches to the strobe head and the other end attaches to either your housing (usually not with compacts) or your tray (a separate item). Adjustable arms are very good to have because you want to control your light as much as possible and this includes the angle the light is coming from. I use locline arms. ULCS are awesome, but pricey arms. Ikelite and several others make arms, too.
You need a tray. This attaches to your housing and then you attach your arm(s) to it. Usually the tray will also have a handle (you may need to buy this handle separately depending on the make of tray). The handle is usually more comfortable than trying to hold the rig just by the housing. Sometimes the strobe arm can be used as a handle so you don't need to buy an additional one.
You need a cord. This attaches to the housing and to the strobe head and is how the camera tells the strobe to fire. There are two main types - fibre optic and hard wired. Make sure you get the right cord for your housing. Most compact housings only allow for the fibre optic; only if your camera has a hotshoe on top AND your housing allows access to it, can you use a hard wired synch cord. DSLRS usually take hard wired synch cords, but the SeaTool housings are an exception and can take fibre optic. Instead of the cord, you can also use a remote trigger system, such as Ikelite, but this can be a bit more limiting as the light from your internal flash must trigger the remote flash. I've used this method with several compacts and found it works just fine, but it is more fiddly than a cord.
So how to you figure it all out? It's not really that hard ultimately, though it seems daunting now. Read a little bit, narrow your choices down.
You need batteries. Rechargeables are the way to go.
Contact a reputable underwater equipment dealer and talk with him/her about your choices and options and budget. He/she will be able to guide you to choices that will work well in a complete system. I use
Reef Photo & Video!, The Underwater Photo Pros for a ton of underwater stuff - they have an excellent selection and are extremely knowledgeable and helpful. I also use Yuzo in Japan as he also has a good selection, fantastic customer service and great prices on some items - his contact details are in the Sticky (follow the pink link in my signature).
