In the meanwhile more and more often companies are offering ultrasonic antifouling systems. Mostly for a lot of money. I met a sailor who is happy on his 45” glassfiber sloop with two pieces of transductors. The PCB and the electronic parts you need to import from an electronic company in Australia. A customer testimonial and the price for the set and how to get, you can find here: http://www.jaycar.com.au/productView.asp?ID=KC5498
An exactly description in a well known electronic magazine, is here: http://www.siliconchip.com.au/Issue/2010/September/Ultrasonic+Anti-Fouling+Unit+For+Boats%2C+Pt.1
The sailor I met, was happy about the parts. Of course, the boat is still painted with one layer of antifouling paint, but the paint is lasting longer. My boat is out of the water now, I need to renew the antifouling. The price of nearby 300 USD in Trinidad and my need of 5 liter is around 600 USD. For one layer. A reason to think over about the 250 Australian Dollar per transductor.