I bought Alexandra Heminsley's book "Leap In: A woman, some waves and the will to swim "on a whim, as I'd heard her talking about on the radio, and as a regular swimmer myself, it sounded interesting, and so it proved. She details her desire to not just swim, but to swim in the sea next to her Brighton home, and takes in other open water or wild swims as well. It also adds to my small collection of books about swimming - or perhaps that should be the literature of swimming, as there is nothing in the form of training manuals. There are a couple of guides to open water swimming, which in my young days was just called outdoor swimming. Brought up partly on the isle of Wight during the 1960's, swimming was for the majority of people a summer activity, in the sea or in the few open air pools on the island at that time. My sister and I were both in our respective school swimming teams, but training was fairly minimal, and we did quite a bit of leisure swimming in the sea during the summer. I've since swum in rivers and lakes as well as the sea, but only as a leisure swimmer, not competitively. However these days I mostly swim in pools, indoor or outdoor whenever possible.
Alexandra Heminsley's book is enthusiastic about open water swimming and I have to admit it is a lovely way to swim, but not always accessible She includes several tips about how to swim in open water, recommends some places to swim in the open air and also has tips about equipment to use while swimming.
There are some guides as to where to swim outdoors: this website has links and maps to hundreds of places in the UK and beyond: http://www.wildswimming.co.uk/.
Go on, leap in and enjoy the water.