I haven't used the Nauticam housing yet, only the Fantasea one (which I think there's a pretty strong case for, as the crazy light weight, and ultra compact size of the Fantasea housing seems a good philosophical match for a tiny camera like the A6500, and you're not really giving up anything ergonomically either).
1- I want to use my actual Ikelite strobes DS160 and DS161. It seems the TTL will work only with fiber optic cables, am I wrong?
From my research into it, there doesn't appear to be any method to extract TTL metering from the A6500 for underwater shooting for the time being.
You can use fibre optic to trigger the DS161 from the A6500 (either using the pop-up flash, or an LED trigger, but you'll need the Ikelite fibre optic adapter for that. I'd recommend the direct fibre optic adapter, rather than the slave trigger with a cable adapter (as I had serious issues with the converted slave trigger not responding to any of my fibre optic cables, whereas the dedicated fibre optic adapter worked flawlessly).
2- If that is true, there is no Nauticam led trigger to avoid the recycle time of the internal camera flashlight.
Do someone knows if the Fantasea one (http://www.fantasea..../it.A/id.748/.f) will WORK and FIT into Nauticam A6500 housing??
Couldn't tell you if the Fantasea trigger fits the Nauticam, but it's very compact and an excellent piece of kit. The added advantage of the Fantasea trigger, is that it will let you shoot at up to 1/250 shutter. Whereas the internal pop-up flash limits you to 1/160.
3- If in the future i want to replace the Ikelite strobes for Inon Z330 or Z240: can I use the same fiber optic cables? do i have to purchase anything else?
You could technically repurpose the same fibre optic cables, but you'd have to switch the connectors on them. The Inon's have a different connection to the Ikelites.
Edited by Grug, 23 January 2018 - 07:20 AM.