I don't think it will pose a threat to how the Ship navigates as much as autonomous fleet management and trading based on data on the positioning of goods carried on ships. Ships will navigate based on sensors onboard, as well as different satellites that are not in the GPS system such the AIS satellite systems.
But yeah, this will be an interesting topic to follow. There will most likely be tens of thousands of new smallsats in orbit serving different purposes within the next ~10-20 years. So then companies managing big fleets or coastal safety will own their own satellite constellations which they can encrypt themselves. Making it a lot harder to hack.
It could also affect private boats, yachts, and smaller commercial vessels. Commercial grade GPS navigation units are cheap and therefore popular. As other options and systems come down in price, then this type of threat diminishes.