Local beaches were fenced with barbed wire and heavy guns were pointed out to the Solent.
But these obstacles were nothing compared to the daunting defences guarding France's Normandy coast during the Second World War.
In the months before D-Day in 1944, gun placements, minefields and sea defences were installed by the increasingly nervous German command.
And yet during that tense period, a group of crack commandos who had trained at Hayling Island left for France on a top-secret mission.
On a small boat they managed to get all the way to the landing sites off the beaches and survey the seabed and shoreline.
Without their efforts, the Allied landings might never have worked and the war might have dragged on for many more months.
Hayling Island researcher Robin Walton and colleagues from heritage group Discover Hayling have now tracked down a survivor of the Combined Operation Pilotage Parties – or COPPs – who were kept completely secret until the late 1950s.
They were a team of 174 men, including specialist marines trained under navy Lieutenant Commander Nigel Wilmott at Hayling Island Sailing Club.
Their aim was to reconnoitre and survey beaches, and they were known only to a handful of commanders in the Allied set-up.
Major General Logan Scott-Bowden, now 89, was one of the heroes to sail to Normandy in a miniature submarine on New Year's Eve in 1943.
Then a 24-year-old major in the Royal Engineers, he boarded a motor gunboat at Gosport with his companion-in-arms Sergeant Bruce Ogden-Smith, who died nearly 20 years ago.
Winston Churchill had apparently suggested the date because he believed the Germans would be far too busy celebrating to notice activity on the beaches. And he was proved right.
Mr Scott-Bowden says: 'Our mission was to reconnoitre the Gold Beach area around Luc-sur-Mer. A few miles from our target, we changed into our bulky rubber swimsuits, strapped on heavy bandoliers, backpacks and weapons, and transferred to a small inshore craft which took us to within a quarter-mile of the beach.'
They then swam ashore, because the D-Day planners needed to know what lay beneath the sandy beaches. Aerial photographs and knowledge of local conditions indicated that beneath the top layer were pockets of peat bog, which could spell real trouble for tanks and other heavy armoured vehicles.
These were the remains of ancient forests, submerged thousands of years ago, and were the focus for the mission.
The pair took core samples along the beach with metal augers, storing them in special containers for analysis back in the UK.
Where they didn't have enough space, they used condoms to store soil samples safely.
Mr Scott-Bowden says: 'As we swam back through heavy surf towards our rendezvous point, I thought my companion was in trouble when I heard him shouting.
'But when I turned to help, he wished me a happy New Year.
'I told him to, "swim you b******d, or we'll land back on the beach".'
They battled fierce weather conditions on the return journey but were able to pass on vital information for the D-Day planners at Southwick Park near Portsmouth.
When the Americans heard of the mission they asked them to survey their own landing sites.
Mr Scott-Bowden says: 'This time we boarded an X20 midget submarine at Gosport, and were towed by navy trawler to within a few miles of the French coast.
'We then spent four days on the seabed, and three nights surveying the beaches near Omaha Beach.
'When we first approached the target area, we discovered our way was partially blocked by a French fishing fleet, complete with enemy guards.
'As we threaded our way through their nets, we raised the periscope and I found myself staring into the face of a German soldier perched close by, on the stern of the last fishing boat, puffing away on a pipe!'
By day they surveyed the defences through the periscope, and each night Scott-Bowden and Ogden-Smith donned cumbersome rubber swimming suits and swam to the shore – all while dodging German searchlights.
Each man was weighed down with a heavy kit of shingle bag, brandy flask, lead weight and underwater writing pad, along with a service revolver, knife, trowel, torch and a dozen sample tubes.
The samples were even collected from the beaches under the noses of the sentries.
The team of five had been in the tiny submarine for five days, and were welcomed as heroes when they arrived back at HMS Dolphin in Gosport.
Mr Scott-Bowden says: 'The welcoming party took a step backwards when the hatch was first opened, and they received the full blast of the noxious fumes trapped inside our sub!'
Winston Churchill, right, who later visited Hayling to view rehearsals for D-Day, asked to meet the two men and personally gave the order for them to be decorated for their bravery during the vital missions.
A small exhibition about COPP is now on display at Hayling Library.
COPPS AROUND THE WORLD
The Combined Operation Pilotage Parties (COPP) teams were set up in the middle of the Second World War to provide undercover information about targets around the world.
They were incredibly secret, with only a handful of officers knowing who they were. Even after the war they remained shrouded in mystery.
It was only when an American researcher began writing on the topic in 1959 that the extent of their role became known.
The first official COPP team was sent out to check Sicily in January 1943 in preparation for landings in March.
These searches resulted in the death of five men and seven captures out of a total of 16.
The British concluded that the men were ill-equipped and ill-trained and that without proper training and equipment all COPP missions were doomed to failure.
So Lord Louis Mountbatten demanded that the Chief of Staffs planning the D-Day invasion give the COPPs their full backing. They did that by issuing an 'ace of trumps', a verbal code making training and equipment requisition a top priority of the war.
Giving the teams the priority paid dividends, which was shown by improved training and better equipment.
AT THE HEAD OF THE NORMANDY LANDINGS
The COPP teams laid the ground for D-Day, but they were also at the front of the actual invasion.
On the morning of June 6, 1944 the midget submarines HMS X20 and HMS X23 were the closest craft to the French coast as the vast armada headed over from HMS Dolphin in Gosport to carry out the hazardous Operation Gambit.
Brave frogmen returned to the previously surveyed areas in the subs and laid poles in the seabed which beamed green lights back towards the oncoming ships.
The lights were not visible from the shore, but the servicemen placed themselves right at the head of the attack to make it a success.
Lieutenant George Honour, who won the Distinguished Service Cross for his bravery in X23, said he was shocked by the size of the Allied fleet.
He said: 'It was unbelievable. Although I knew they were on our side it was still a frightening sight.
'One can only imagine what the enemy must have felt, waking up to this awesome spectacle and knowing that they were the targets.'