If you want TTL with your Inon strobes, you will need an external TTL converter from Heinrichs Weikamp (or Sea & Sea if you use their housing). This will sit between the sync cable to the strobe and your housing connector.
For Aquatica housings MAKE SURE you specify that you will be using a TTL converter as the connectors for manual and TTL convertors are very different. If you connect a strobe to a standard (manual) Nikonos connector on the housing and try to use TTL, there is a possibility your camera will freeze. This is why on housings like SEa & Sea, there are 2 bulkhead connectors. One is for manual strobe control, the other is for plugging in a TTL convertor if you choose to use TTL.
With Ikelite, you will get TTL if you use their brand of strobes. If you don't you will also have to get a TTL converter.
Sea & Sea will be releasing their polycarb housing for the D90 soon. That housing will have the option for optically fired TTL. This I find is the best choice. No expensive cables or convertors and no failure points which can flood. Nexus (Anthis) also has optical connections for Fibre optic cable fired TTL. However, I am unsure of when they are releasing their D90 model (D90F-SO I think). The housing is aluminum and Nexus has a history of pairing up with Inon products. You will find that although the Nexus and Aquatica housings are much stronger than Sea & Sea's and Ikelites housings, they are also much more expensive and heavy.
My approach would be this. Get a fibre optic connection for TTL. This means either the Sea & Sea or the Nexus housings. Whichever you choose will depend on your budget and willingness to carry a heavy housing. I am a Sea & Sea fan but Nexus does come a close 2nd. In this case, I would certainly get the Nexus (budget not withstanding). This is because Sea & Sea's D90 offering is polycarbonate and Nexus is machined from aluminum. Nexus also has port adaptors which will allow you to use other brands of ports. I also like Seatool housings which also offer fiber optic connection but they have no D90 model in the works as yet I believe.
For lenses, I believe the 11-16 will require the use of a diopter to focus properly underwater. I think a 4T will do but you should check. The Tokina 10-17 fisheye is a much better wide angle lens underwater in my opinion. AussieByron (Mark) seems to agree....as do many other wide angle (WA) shooters. It focuses SO close that you can literally have the dome port touching your subject and still fill the entire frame while keeping everything in focus. You WILL need a dome port for any proper WA work.
The 60 is my favorite lens to use. It is the most versatile and probably the easiest to use. I can capture macro shots, fish portraits or even accomodate whole mid sized fish. If you are more interested in general aquatic life mid sized to tiny, this is the lens to use. If you are into seascapes, schooling fish, really large pelagics and the occasional close focus wide angle shots, the Tokina 10-17 FE is your best bet.
