Commercial receivers even when they receive only L1 C/A have anti-spoofing capabilities. The Horizontal Alert Limit will be rapidly exceeded by the Horizontal Integrity Limit and annunciate loss of integrity. Most receivers usually notice a collective shift of several hundred meters. Due to ITAR, if the apparent position shift exceeds a speed averaging above 800 knots, the GPS receiver then assumes it is guidance for a weapon and "locks up". INS is usually a good back up, if you have a decent installation. Civil aircraft without a hybridized GPS/INS will still be able to maintain an RNP-10 for at least six hours. This doesn't include the latest commercial receivers that will be able to use L5 freqs to determine ionospheric scintillation from each satellite, which will only improve Horizontal Integrity at the receiver. Even with SBAS, there is only a guarantee for 99.5% GPS coverage for en route capability (RNP 2.0 or better) within CONUS. All newly certified aircraft will have to meet this performance requirement. This also affects things like ADS-B and ADS-C, which absolutely require GPS Position/Nav/Time.
So while SAASM is great and all, it's only for the military really and the second frequency from the L2 band, helped in finding the ionospheric scintillation constant for the emitting satellite.
Kinematic GPS is ok, but it takes forever to align in high precision mode and typically, it's not made for the rigors of performance based navigation, so the vendors all opt for a very good L1 band solution on civil aircraft. For ships, I would hope they use backup inertials to supplement GPS like some of the higher end new aircraft do.