have had lengthy discussions on this subject. Consider:
A separate AIS antenna can be tuned for the two specific (adjacent) frequencies used for AIS, while a general VHF antenna is designed for the lower middle of the VHF spectrum.
With a switcher, you miss what's going over the other system when you transmit. Some AIS information is sent only once every 6 (?) minutes.
If you have two antennas and lose one, the other can be plugged in to use in an emergency
If a switcher fails to isolate the other radio
when you transmit, it is fried.
If thirty minutes warning of an impending collsion is enough for you, your AIS antenna can be mounted lower.
When you go to an AIS B transceiver, you will want separate antennas to maximize your investment, and you can use the coax you pull today.