What radio gear would "you" recommend for HMS Upholder
A 40mhz system with four channels or more. I would recommend getting at least 6 channels, as you may wish to add extra functions, or make another sub later on. In terms of make, Multiplex, Futaba, Hitec and JR are all very good. Multiplex and some of the JR radios are particularly worth looking at, as some of them offer synthesized tuners, which means you don't have to fuss around with crystals. Purchase a decent receiver- signal quality is greatly reduced underwater, so you want a receiver that can filter out the muck. Computer sets are a nice luxury, but really far from necessary for model submarines.
What Control system should I use?
I use microgyros ESC's for brushed, and Castle Creations for brushless. Mtroniks controllers are also widely used in submarines.
Importantly - Dive components - I am clueless to this at the moment and somewhat totally mind boggled!
Depends on your preferred ballast system, depth of pocket (if purchasing rather than scratchbuilding) and level of skill. For instance, piston tank systems are quite expensive if purchased, and difficult to make unless you have access to a lathe.
Other items like pipe for the pressure hull, seals for the motor and control shaft, are readily available. Seals for control rods can be scratchbuilt if you prefer. However items are available quite inexpensively. Bellows are also often used to seal control rods.
Some people like to use acrylic for the pressure hull, as it's transparent. Alternatives to this are PVC pipe (e.g. waste pipe) and polycarbonate. The latter is rather expensive, so is used less in the UK. PVC offers the same durability as polycarbonate, but is usually opaque, it's probably the cheapest option.
Building the Sub it's-self? How do "you" keep your 2 halves together in the wet stuff?
There are a few methods. I like to use a machine screw to hold hull halves together. You can often disguise this as a bollard etc. Some people use ball catches, but these can come aprt sometimes in the water, so I'm not a fan. Small pegs made from brass rod help keep the two halves aligned and prevent the hull from spreading, which polyester laminates are prone to if left unsupported.