There are no known reasons why a woman known to be carrying GBS should not be able to have a water birth. The Royal College of Obstetricians & Gynaecologists’ group B Strep guideline states:

7.5 Can GBS‐positive women have a water birth? (click here for full guideline)
Birth in a pool is not contraindicated if the woman is a known GBS carrier provided she is offered appropriate IAP.

As for all women know to carry GBS during their pregnancy, they should be offered intravenous antibiotics from the start of labour and usually at 4 hourly intervals until delivery. It is not a good idea to get the cannula (which delivers the antibiotics to the mother) wet, but this can be managed – specially designed waterproof dressings are available which keep the site sterile and dry whilst still enabling the health professional to monitor the site visually.

You can read an article about water birth and GBS, Water Birth for Women with GBS: A Pipe Dream?, which was published in The Practising Midwife in 2007 by clicking here.