You may already know this one, but I don't know if it specifically addresses BMI. Often high BMI is used to rule out water immersion, because staff may have difficulty assisting in an emergency situation:
Cluett ER, Burns E (2009) Immersion in water in labour and birth. Cochrane Database of Systematic Reviews, Issue 2. Art. No.: CD000111. DOI: 10.1002/14651858.CD000111.pub3.