Hello
1) Batteries: 2 x LR44 1.5V or equivalent. Another option is one Lithium 3V, which I think is the CR 1/3N
2) The brackets are where the focus plane is. In other words you have to "touch" with the brackets whatever you are going to capture. This fact makes the macro sets for the nikonos almost impossible to use with live subjects.
3) Hot shoe viewfinders: There are 2 plastic view finders and one optical. The plastics are: one for the 28mm and another for the 35 & 80mm. As Ellis said, practice (the 80mm is very difficult to use this way). The optical viewfinder is to be used with the ultrawideangle 15mm (there are two models of the 15mm, the earlier is a symmetrical lens and the newer is a retrofocus type lens. As far as I know, the design change was to allow TTL mettering, which was impossible with the symmetrical lens)
4) The 35 mm can be used both above or under water. What is going to change is the angle of view. Also underwater the perceived distance is not equal to the measured distance, focus should be set with perceived or compensated distance. If you measure distance with a tape, you should compensate by setting the lens 25% closer, so if you measure 4 feet, you should set the lens at 3 feet. The perceived distance underwater will be also 3 feet.
In these links you can find the manual:
Nikonos IV-A part 1 ;
Nikonos IV-A part 2Keep in mind that the Nikonos cameras have been discontinued several years ago and the O-rings are no longer available new. Also there are only few user-serviceable o-rings (lens, back, battery cover, flash connector). There are other o-rings (i.e. film rewind and focusing and aperture knobs in the lenses) that should be checked and eventually changed, but I'm not sure if there is anybody still servicing them.
The o-rings are not standard measure, you can't just get them at a hardware store.
Having said that, I would recommend not using it underwater, at least in salt water. It only takes a single incident of salt water getting inside to kill a nikonos, I've been there. You could still use it above water with the 35mm (IMHO a good performer) as a great all weather camera