When traveling on a commercial aircraft the pressure is roughly equivalent to that of 8000 feet
As such there is a reduction compared to sea level.
This pressure difference is unlikely to burst the seals or crystal of any watch
(with a Seadweller you may get the release valve burping to equalise pressure as you ascend - Unlikely)
Reduced pressure only becomes an issue with watches when flying at high altitude without pressurisation, something most people on the planet are unlikely to do
If you do find yourself at 30,000 feet in an un pressurised aircraft your watch is the least of your worries.