As OP hasn't replied yet, I'll take the liberty. The biggest issue is interference (jamming) by malicious actors. This is especially common in the eastern Mediterranean [1]. Earlier this year there have also been reports of interference on Finland's border with Russia [2].

Of course one could imagine other scenarios where GPS might become inaccurate or unavailable on a larger scale, such as an attack on the satellites themselves.

[1] https://safety4sea.com/us-marad-gps-interference-incidents-r...

[2] https://www.traficom.fi/en/news/unusual-amount-reports-about...

(my guess is that _moof is a navigator with experience in the eastern med sea)