If you prefer to carry your Nikonos around your neck this adjustable plastic strap is perfect in or out of water. However it is important to carry replacement O rings for the main user changeable O-rings in case these are abraded or nicked during normal pre dive preparation. O-ring kits include silicone grease and are available for most Nikonos bodies and Speedlites. This is the plug that seals your Nikonos from water damage and dirt when you don't have a strobe plugged in. It is very small and very loseable.
Spares are highly recommended.
Batteries The Nikonos IV-A and V are extremely economical with their batteries. However, they are button cells which can be hard to obtain at dive sites so it is wise to carry at least one spare set.