Anyone afloat on Plymouth Sound one Sunday last September might have been forgiven for thinking that time had suddenly been reversed. For, chugging across the Sound among the smart yachts and pleasure boats of weekend sailors, could be seen the SS Advance: 220 tons of genuine coal-burning steam-snorting nostalgia straight out of the 1940s. And stowed prominently on deck was an ancient air pump and three sets of gleaming hard hat diving gear, a sight which had probably not been seen at sea in that part of the world for 30 years or more.

The SS Advance was the perfect platform for what turned out to be a dive into history - an event organised by a small group of local enthusiasts, of which I was fortunate enough to be a member. The idea was for six of us to make an open water dive in a genuine Siebe Gorman Standard Diving Dress, a rare opportunity in this day and age.

Nigel Boston, owner and skipper of the ship, had made her available for the day in return for a dip, and was looking forward to the experience as much as any of us. All that remained was to find the right spot. In a stiff north-westerly that spot turned out to be just off Fort Picklecombe, Bovisands opposite number on the western side of Plymouth Sound. Once there, anchored in 10m of water, and with the Jolly Roger flying in preference to the A flag, we were ready to begin.

The equipment for these dives was supplied by Ray Ives and Mick House, co-founders of the Hard Hat Divers Company, and every item was the genuine article - right down to the huge twin-handled air pump (circa 1872). Under Micks expert guidance, many divers have already made the transition from mask and fins to helmet and hose in Bovisands training pool, but this was to be the first time this equipment would be used at sea by amateurs.

In common with Mick and Ray we had every faith in their diving gear, ancient as it was. As they took pains to point out to us, the combined ages of the six participating divers came to 293 years, whereas their oldest piece of gear clocked in at a youthful 123 years! It was certainly food for thought.

After dressing and posing for the obligatory helmet-on-knee photographs, Mick was the first down to test the system. I accompanied him in conventional gear and watched as he vented the suit and made his descent to the bottom, to stand like a diving dinosaur among the kelp. Real as he was, it was hard to believe the pictures I was seeing through my cameras viewfinder.

I could also see that he was chatting away to the surface on his helmets intercom. Had I been dressed in standard gear as well, we could have held our own diver-to-diver conversation simply by touching helmets together and talking. However, as I soon discovered, a modern neoprene and bone head is useless for eavesdropping on a hard hat diver. Although looking forward to my dive in the suit, I found it equally fascinating to watch the efforts of others from the outside. A Standard Diving Dress in action, both on the deck of a ship and on the seabed, is a wonderful sight, conjuring up pictures from the past as few things can. It made me wonder what the divers of yesteryear would have made of us and our modern equipment had they been given the chance to leaf through the pages of time as we were now doing.

The process of kitting up in one of these suits for a pool dive has already been described in Diver (Feb, 95). Suffice it to say that doing the same thing aboard ship for a sea dive differs only in one respect: you have to concentrate much harder on trying to look calm and unruffled. For example, theres 3m of awkward ladder to negotiate, even before you get into the water. This is not an appealing prospect when you can neither see nor feel the rungs and are wearing 77kg of equipment.

What if you slip and fall

Then theres the question of buoyancy control. Everything hinges on the correct use of an exhaust valve on the right side of the helmet. Unscrew it to vent and descend, screw it closed to increase buoyancy. It all sounds simple, but what if you get it wrong and end up on the surface as the days first fully-inflated Dunlop advert

It doesnt bear thinking about.

Fortunately all goes well. As expected, the hardest part is getting down the ladder, but once in the water I feel fine. A shot-rope is within easy reach of the last underwater rung, and it is a simple matter to swing onto this and operate the helmets exhaust valve. Down I go, hand over hand, pausing only to clear my ears as the pressure builds; then Im on the bottom, up to my knees in kelp.

Outside is Dave Peake, safety diver and cameraman for the day - and the strange thing for me is that he feels outside. As he swims past my saucer-sized window and waves a hand I realize that he is no more my buddy than the man in the moon. If I have a buddy at all its the tinny voice in my helmet asking if Im OK. I am, and apologise for forgetting to say that Id arrived on the bottom.

Walking around on the seabed is not nearly as straightforward as it looks in Hollywood movies. There are a number of basic techniques, but each depends on the correct adjustment of buoyancy. Too much and youve no purchase underfoot; too little and you tend to stagger and stumble along. However, once mastered it allows you to hop, skip and jump with the best of them.

Perhaps the best thing about this type of diving is the fact that while youre tramping about down there you dont have to worry too much about navigation. Your hose and lifeline always lead back to the ship. Oddly, one of the most difficult things to master in Standard Dress is the simple business of judging distance. The one thing which usually bothers us least under water - the fact that objects appear a third bigger and closer than they really are - suddenly starts to cause problems for some reason.

For example, the shot-line eluded my grasp twice, although it appeared within easy reach, and the same was true of a stalk of kelp I tried to grab when first arriving on the bottom. I would not have made such a mistake in conventional gear. I wonder why

There is one thing which remains as true of hard hat diving as it is of conventional sports diving, however. It is simply that time under water goes all too quickly.

The days diving came to an end as the last of us stepped out of the suit and the first of a series of squalls swept in across the Sound, ending the days sunshine. As it happened, these squalls also signalled the end of a long hot summer. What better way to end it

We are currently considering running one or two more dives using old but serviceable diving gear of various kinds. Readers interested in participating should contact Paul Dart on 01752 896018.