They are his body fluid.....water is what makes up most of the fluid in your body. So if Raiton hits him while liquefied, he gets fried.
This definition is correct if you look at it that way. As body fluids means: A natural bodily fluid or secretion of fluid such as blood, semen, or saliva.
Now the question, is what Kabuto doing by natural means? Is he literally using natural body fluids from a human body (Blood, saliva, semen, sweat, etc) to trigger them like Hozuki clan?
By definition it doesn't work out, and I don't think Kishimoto would even use these fluids and demonstrates to his younger readers either.
We have seen this case before with characters with different body fuilds via Kidomaru. His body fuilds can create web silk and also converts it into gold that blocks chakra[
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Nothing implies his body fluid is made up of some unknown liquid.
1. It was implied that it was created[
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] "Its a technigue that I created and after researching and altering..." The context separated his use to Suigetsu (where he stated Suigetsu can transform his flesh to water.)