You will be required to complete a UKSF Swimming test during the first weekend on the selection programme.
You will be made aware of what is required in detail on the day.

However in general you should be a strong swimmer.
You will need to be able to swim up to 1,000 metres, tread water for a period of time and recover a weighted object from the pool floor dressed in combat shirt, combat trousers and belt with pouches.

Train for the Swimming test by including swimming as a regular part of your fitness training programme.
Practise swimming in trunks for half an hour non stop, building up to doing 45/50 laps of a 25m pool.

Then, to increase your strength whilst swimming try using floats.
Try doing four lengths of the pool with a float in your hands to strengthen your kick, and then with the float placed between your legs to strengthen your stroke.