I am not sure about support for your boobs but I did notice that Mamas and Papas sell Maternity Swimwear. They are doing a sale at the moment so they may have some on offer as clothing is dear from there. I have the same problem with my swimming costume at the moment, I bought one that was a size bigger and thought I would be able to use it with my growing bump but it is soo tight now that I think I will need to get a maternity one for when I go swimming.
