hi sade,
im a big girl too, and i fully understand your concerns,
i dont actually go to a public swimming baths, but i do occasionally have a dip in my aunties big heated indoor pool with family, and i wear one of those swim dresses that joannie suggested.
mine's a navy blue and white patterned one, and it looks and feels a treat. i've had many a compliment on how nice i look in it. (and it covers up the tops of the legs, nicely)
i do understand you feeling self concious and all that, but, like everyone else has said, nobodys going to take any notice of you.
and once you're in the water, well you'll be fine.
maybe have a towel handy for when you step out, just so you can 'casually' drape/hang it infront/around you till you get to the changing rooms. but if you get one of those dress cozzies, you wont need it.
enjoy your swim, sadie :o)