Still there on my computer. Maybe a fault at your end or down briefly for maintenance?
For anyone that says it's "working", and is on a PC, can you try "ipconfig /flushdns" from a command line, then try again from a fresh browser.
If you ping www.navionics.com from both UK VPN and Greece, do you get the same IP address?
It looks to me as if they have failed to pay the bill or allowed the registration with their provider to lapse.
I can get it to work if I manually add a hosts entry for 107.20.170.12, otherwise I get:
C:\Users\Dave>ping www.navionics.com
Pinging www.navionics.com [208.91.197.132] with 32 bytes of data:
Reply from 208.91.197.132: bytes=32 time=140ms TTL=241
Reply from 208.91.197.132: bytes=32 time=143ms TTL=241