I would investigate carving figures out of some polystyrene foam, then give them a coat or two of epoxy resin to toughen the skin a bit. Very inexpensive and a bit of fun to make.
Models of chariots tend to be a bit top heavy, and the owners have often applied a keel to get them stable, so I would make every effort to keep the figures as physically light as possible. Using foam the figures should actually raise the centre of buoyancy a bit. Action men would probably be neutrally buoyant at best, as they tend to be quite heavy.
I've seen suits made up using various fingers cut off rubber gloves. Sadly the one pictured suffered some reaction as can be seen by the cracking on the suits.
Although most models seem to have the suits modelled in black, a visit to the submarine museum at Gosport revealed that the suits were more of a tan/beige colour. perhaps they've faded with time and tide, although no doubt they darkened once wet.