Most modern solid-state-output-circuit transmitters are smart enough to reduce their power output when they encounter an antenna
load that is not well-matched. Whether the specific device on your boat
has that feature is unknown to me.
A general upper limit for VSWR before indicating an alarm
condition is perhaps 2:1 or higher, or perhaps even 3:1 or higher.
If the antenna for the AIS transmitter were not specifically tuned to 162-MHz and was instead just a general VHF Marine
Band antenna that would be tuned to 157-MHz, the general 157-MHz-tuned antenna would likely have a rather high VSWR at 162-MHz, and this may be the source of the problem.
On the other hand, if the VSWR alarm
alert is something new and the AIS transmitter did not previously alert about VSWR, then a new problem in the antenna, the transmission
line, or any connections involved may be the cause of the VSWR alarm alert. An open circuit or a short circuit at any point in the circuit would cause a radical change in the VSWR on the transmission
line as seen at the AIS transmitter trying to input power into the transmission line.