There are a good many weekly swimming sessions throughout training, it is becoming an increasing feature because it is excellent non-impact CV exercise which culminates the Battle Swimming Test (below) at week 9. If you fail, you undertake remedial training and it must be passed by week 17.
This is an old video, but fairly accurate I believe.
The long and the short of it is the clue to the job is in the word "Marine", meaning 'of the sea' and swimming is very much part of the job.
Hi my nod is in week 15 ish and they have done a lot of swimming with it increasing in duration and pace. As already stated it' a very good low impact workout as well as being an obvious necessity for the job. Advice would be not to neglect it in training either pre prmc or indeed pre r.t. but build it into you phys.