Jonathan,
The problem is more the wet swimsuit favoring the growth of candida that the
mother already has.  Chlorine is pretty nasty stuff, and if in high enough
levels, kills almost any bug.  If mom is prone to thrush, she might swim, then
change into dry clothes quickly, rather than laying in a wet suit.  There is a
new swimsuit material that is purported to dry quickly, perhaps that would
help.
Catherine Watson Genna, IBCLC  [log in to unmask]   NYC