The lack of seams and stitching as is common in leather and fabric footwear means there are no potential points that water can enter a wellington boot and as long as you do not enter into water above the height of the boot there really is no possible way for a wellington boot not to be completely waterproof so long as the structure of the moulded boot is intact. Any wellington boot that is allowing water to enter has usually been damaged or punctured to give water a point to enter the boot.
During the harsh, wet and muddy conditions endured by those who experienced trench warfare in the First World War, the wellington boot became an essential item of personal kit. A medical condition called trenchfoot was prevalent in the First World War,it is one of many immersion foot syndromes but is most commonly experienced when feet are cold and damp for long periods of time. It was experienced by many of those soldiers who fought in WW1 due to being exposed to damp and cold conditions for prolonged periods of time in the trenches.
The trenches were notoriously squalid and damp and optimum conditions for a medical condition like trenchfoot. An advanced case of trenchfoot is signified by the previously numb feet beginning to blister and have open sores although a mild case of trenchfoot would expect to see a full recovery from the sufferer. One thing that all sufferers of trenchfoot agree upon is that it is a very painful condition and has often been described as an intolerable pain by those afflicted with the condition.
The mass scale production of wellington boots became a top priority during the First World War. Soldiers needed to be fighting fit and able to defend trenches from enemy attack not barely able to stand and afflicted with terrible pain in their feet. To meet the British Armies demand for waterproof footwear a total of 1,185,036 pairs were manufactured to keep the army on its feet and fighting fit. Apparently the German soldiers were very envious of the British soldiers dry feet and their wellington boots.
Whether the German Army was issued with wellington boots I have not been able to find out but after the First World war the humble wellington boot has been cited as one of the reasons why the British Army was able to endure and eventually prevail as victorious. Personally I believe the true winners were all the soldiers feet saved from trenchfoot and even ampution all thanks to the humble wellington boot which today we all take for granted and can buy cheaply at your local store.