Swimming is a low impact sport, as it is non weight bearing. This is good news as you are less likely to pick up an injury compared to other sports.
You should not enter your first open water event without having swum previously in the outdoors, regardless of whether you will be swimming in a wetsuit or not. Start off gradually and build up your distance.
You should swim at least two times a week and ideally three (one of these should be outdoors where possible).
To train for an open water swim, you need to think about two aspects of the event:
Being swim fit: the easiest way to become swim fit is to regularly use a swimming pool or lido.
Being acclimatised to swimming outdoors: on the event day it could be cold or a bright sunny day, and the water could be calm or choppy. You should be comfortable swimming in all conditions. It is very different swimming outdoors compared to swimming in a pool. There is no black line on the bottom of the lake to follow, the water may not be as clear, and you must look up to see which direction you need to swim. You will also have other swimmers in close proximity to you.