If you don't have a single-piece swimsuit, do yourself a favour and go shop for one, except with one difference: try and buy it several sizes smaller.
Imagine if you were 10 years old, buy one that size. I still have my old primary school swimming costume. In fact if you want to get really kinky, find out where you can buy swimming costumes for private schools, you know school colours costumes. Tell them it's for your 10-year-old daughter or something.
You'll be very surprised how exciting it can get to have a really tight swimming costume on. It's like a second skin (especially when all wet). By far the best thing is the elasticity of it. Your breasts feel as if they want to pop and break straight through the fabric, and if you're well endowed in that area, the tension tends to pull the costume up, giving you a very pleasurable vaginal wedgie (as odd as that may sound it really is quite exquisite). If not, stretch your body out, raise your hips, or just pull on the straps to give yourself one.
And thats not all. If you want to take it a little further, get a comfortable sized dildo or vibrator (your preference really), and place it inside you using the bottom part of the costume as a kind of 'strap-on' support. That way you have your hands free to do as you will.
I've tried a variety of masturbatory methods, and this is by far the most explosive orgasm-nay orgasms-I've ever had the pleasure of experiencing.
Oh, and I'd love to hear any comments from anyone who's given it a shot. Actually, come to think of it, guys can also make use of this technique, though I think for them a larger size swimsuit would be healthier ;p.