I developed some early GPS software/firmware for use in systems that test the systems that test the guidance systems where high precision is required. So, I know a thing or two about GPS and have some familiarity with ways by which systems may be interfered.
I will say that it is unlikely that anyone here has been subject to GPS spoofing unless they happened to be very close to an area that was actively spoofed. This is done sometimes (though less than you might think) around sensitive areas in war zones around other militarily or politically sensitive zones (e.g. the Kremlin, areas of Syria, the Black Sea, contested waters around China, etc.).
Spoofing is not trivial to do and it pretty much needs to be targeted. That is, it requires a high-powered, directional antenna within line-of-sight of the targeted vessel ... or a transmitter that is very close to the targeted vessel. (It is "technically" possible to do this another way that is less targeted, but impractical. Instead, such methods rely on interference rather than spoofing. Interference is much easier to create than true spoofing.)
Software-defined radios make this cheaper to do than it used to be, but it is still rather limited in its effectiveness.
If a sailor experienced a sustained and consistent effect, it is MUCH more likely due to one of a few other causes.
1) Multipath Error: This is the most likely culprit. This is interference such as from the coastline that causes GPS signals to bounce. It would be unusual for this to produce a consistent effect, but this gets into GPS software/firmware. It is entirely possible that the GPS software simply filters out erratic signals and takes its best guess. (This is a normal function of all GPS software/firmware.) So, to the end user it looks like the GPS is confident about a consistently erroneous position but, in fact, it is simply getting slightly more erroneous signals than valid signals, and showing the user its best guess.
2) Unintentional Interference: There is a long list of things that can unintentionally interfere with the weak GPS signals, including just about anything electronic, in one way or another. I would not expect this to produce a consistent error unless the source of interference was local to the vessel. But, as unlikely is interference is to cause a consistent error, it is still more likely than a private vessel being targeted with a spoofing signal for a sustained period.
3) Intentional Interference: This is sometimes done around sensitive areas but it is very different from spoofing. It will not produce a consistent error but will, instead, produce erratic errors.
4) Receiver Issues: GPS systems are very reliable but, you have to remember that they are made by humans. The reason I developed systems that test the system that tested guidance systems is because humans make errors that can be very hard to notice because they only happen under very isolated conditions. When we design systems, naturally they work most of the time as intended because we test them under anticipated conditions. We don't, however, predict every possible situation. So, bugs get released all the time and, because they only cause problems under very specific conditions, largely go unnoticed.
5) Datum Mismatch: This used to be more common than it is today. The GPS satellite constellation and mapping data/software are not well-standardized. They are actually a hodge-podge of competing standards. This gets a bit into cause #4, above. It is entirely possible to be subject to a consistent mismatch between two competing standards. This would cause a consistent but seemingly confident error. The older the GPS receiver and/or map data, the more likely this is, as a possible cause. Also, this is more likely in certain parts of the world.
The reason that spoofing isn't done much is that it is exceptionally difficult to do and it is only effective if the target does not know they are being spoofed. And, it is very easy to know you are being spoofed. The more high-value the target, the more likely it is they will be aware of the error and be able to easily navigate by other means. It is easier to create interference which has the same end-goal, when the target knows they are being spoofed. That is, when the GPS signal is interfered with (not spoofed) the target is fully aware. So, there is rarely any benefit to true spoofing.
That said, it is now possible for an amateur to spoof GPS if they can get close enough to the target and within line of sight. And, amateurs will do this just because they can, because people suck. It doesn't have to make tactical sense. It is still not trivial, but not that difficult. I could acquire or build the requisite hardware/software to do this in a couple of days. (Though, it isn't the method I would use if I ACTUALLY wanted to cause problems.) There's probably a YouTube video that shows how to do it. Fortunately, amateur attempts will generally produce pretty unimpressive results, so such actors lose interest pretty quickly.
Most well-resourced and informed actors recognize that GPS interference/spoofing has really limited tactical or strategic effects. Some state actors do it more for psychological impact or "flexing" than anything.
