After much searching, I found your question.
I don't have any experience in this application of Plasma to comment from.
What I can do is give you some points to consider.
Plasma energy is composed of multiple spectrums. Electromagnetic, radio frequency, audio frequency, scalar waves, etc...
Each has a distance limitation of effect, and each will loose power as distance grows. How water will attenuate the signal of each spectrum I'm not 100% sure of in my head.
Furthermore, frequencies can not devitalize large scale organisms. The human cell for example is too large.
Sea lice is a complex organism. In order to kill it using plasma, you will need to kill all of their gut microbiome, and this will only work if they rely on it to survive -- which is most likely the case.
However, in the case of using Rife against fleas, we are only targeting a controlled area -- the house.
This technique would not work out in the yard as there are too many families of fleas, and one would only be able to target a subset of them at any given time.
For what you want to do, Radionics may be a better modality to employ.
For more details, please check the link: