Build up your tolerance gradually.
A few years ago I was on holiday in Portugal in about March time and the hotel had an unheated outdoor pool. In summer it would have been amazing but it still wasn’t much over 20° during the day so the pool was quite cold.
First day, sort of sat on the edge dipping legs into it. Then stood on the shallow end (water up to mid thigh), them gradually waded in a bit, finally swimming and by the end of the week we could happily dive into it.
Had to do cold showers at uni for a long weekend after our boiler broke down and again it was just a case of building up tolerance. A few seconds at first, in shower as fast as possible and jump out. By Monday morning I could stand in there for a few minutes. It wasn’t “pleasant” but it was tolerable.