EDIT DUE TO QUESTION EDIT: Well, how "common" are solo sailors on marathon journeys? I did a quick search, and wikipedia gave me a list of 49 names. Assuming that this list is most likely incomplete, we still have a very small sample size. Again, I think there are more Navy SEALS than there are solo sailors. The SEALS display this as a common technique because they are forced to do it. For the solo sailor, this would equally likely be the case since there are many things they need to do while navigating a boat.
There is a community of single handed sailors that discuss sleeping, and the "not being in control" part seems to be a frightening thing for them. Another consideration is that there are no navigation light signals that indicate a sailor is getting sleep. While the big ocean theory may hold for most, it would be a bad thing to get hit by another boat if you are sleeping.
Another factor to consider is that as technology changes, the sailors may be able to get more sleep and only get woken up if a condition warrants their attention. Many folks (myself included) don't consider things like auto-pilots for boats, and a proximity warning system. So while the sleep deprivation may have been a normal technique to deal with the requirement for attention, that requirement may be getting solved by technology, thus reducing the need to take this drastic sleep measure.
This 2002 trans-Pacific race features the plans some of the sailors were going to use, and they say things like:
The plan is sleeping during the morning and sailing all night with naps interspersed here and there.
So I would say that it was absolutely common from the tales of those sailors, however is falling out of favour due to technology.
ORIGINAL ANSWER: Asking "Is that possible?" may not be the best way to phrase this question for the format of this site. That said, there are well documented cases of Navy SEALS during "Hell Week" training (Or BUD/S as they call it), where they gather only a total of about 4 hours worth of sleep during 5.5 days.
Here is a document (PDF) where they talk about the sleep deprivation in a study on the effectiveness of caffeine as a stimulant and its effects on marksmanship.
So, there, it's "possible"...