There are two questions
I have not tried the ultrasonic antifouling systems, however I am familiar with the physics. The systems I have seen advertised would not give me any concern about health effects, as they are so low-powered that it is doubtful that there would be any.
A bigger question is the effectiveness. I presume that the physics of ultrasonic waves interfere with marine organisms' chemical process whereby they attach thin filamentary threads into the surface, and then build on those. Thus they might be fairly effective right over the transducers. However, because the systems only have two or at most 3-4 transducers glued to the inside of the hull, the strength of the ultrasound field on the outside of the hull must vary greatly with the distance from the nearest transducer.
In fact the sound field strength, and thus presumably its effectiveness, falls off as the square of the distance from a transducer, and so the transducers must either be greatly over-designed to have a larger coverage area, or the antifouling will not be effective except right over the few transducers. So I would expect that the system coverage will be spotty at best, though I have no direct experience in using them.
Therefore I would recommend asking the manufacturer of any systems that you may be considering to buy a few questions such as: How even is the coverage over a hull of your length? Does the ultrasound deter all growth, or does it work on some though not other organisms, and if so which ones? Does it prevent slime growth? How well? And the summary question: Show me data on effectiveness compared to copper paint, at various distances from a transducer in a typical application.
A legitimate manufacturer who makes an effective product will have these data, and will be eager to share it with you to make a sale. If he does not, go elsewhere or stick with antifouling paint.
