I've often pondered this myself. But I've come to the conclusion that for me, I'll stick with selling vintage swimwear and leave the designing to the experts. There are some wonderful informative articles I've found though that I'd like to share with those of you who are aspiring swimwear designers.
How to become a swimsuit designer.
Facts and figures on being a lingerie and swimsuit designer.
And just for fun, design your own swimsuit.
And if you're already savvy at sewing pop on over to Pattern School for a good dose of articles on selecting swimwear fabric and sewing information.