I remember there were three phases to getting into the North Sea in April wearing only trunks. The first was wading in up to thigh level which was teeth shatteringly cold. Then there was the first slap of a wave against the gonads which made you gasp and look around at your pals to see if anyone else had anything but a stoney look of resolution on their faces. Then there was the 'men only' let's just go for it dive from a standing position which literally did take the breath away. After that it was fine, in fact it was colder to stand with your shoulders exposed to the air than it was to keep everything but the head underwater.
Then we used to watch helicopters with men in survival suits and dinghies fly over just in case they came down in the same bit of water that me and my mates spent hours in wearing just trunks. Never figured it out.