Comfrey prefers water, but it will survive if you cant get to it, once it has enough root base. There was a piece of root I planted and forgot about until I saw it come up the next year -- but that wasnt full sun. Hm, there is another one forgot about, grown over by blackberries for a few years, blackberries aggressively taken out by workers, with no thought there might be anything there to save. A pile of wood chips dumped. Then, the next year like a phoenix rising form the ashes, comes that comfrey plant.
It does not rain here for 6 months of the year, sometimes longer like last year.... But, if you want to get multiple cuttings, and nice large growth, water it.
This is why I dont have the spread by self seeding type. ANd, this is why they say to think hard about where you plant your comfrey as it will always be there.