Look at all housing specifications closely. Eventually, no matter what you buy, you will probably want to add a strobe. Some housings do NOT come with the ability to connect an optical sync cord (none for the TG-6 have electrical sync options).
The PT-059 is an excellent housing and does work with the on-camera flash. Fair warning; in our murky water it's really only good to about 12in, in other words acceptable to very good for macro work, but not at all good for longer range work (the on-camera flash, that is).
The PT-059 does come with a mask / mount for optical sync cables, and can handle the two common connections (bare cable and plug-in). It has blanking plugs for the hole you don't use if you don't plug in two cables.
I've used on-camera flash for several dives now and it's really very good for macro, especially on high f-stops (to black out the background).
I also tried my Kraken 3000 ring light with less than enthusiastic results:
pro - it fits the housing if you buy a 52-67mm step up ring, which is really inexpensive
pro - it does work, and will trigger the 'strobe' optically
con - it blocks the on-camera flash so you must use it
con - it's so close to the flash plate that you can't add any normal sync cord plate. I sort of jammed the cord into the slot on the top. It worked but I can't say I was happy about it
BIG MINUS - it gave good 'video' light for macro, but the strobe function is just too underpowered to deliver really great photos. Like the on-board strobe, it's range is very limited as well.
Tomorrow I'm trying my new Inon S2000 and can report back.