Good to know.
You have a few options.
One is to run the coax cable to the top of the mast and install the antenna there. You will want to make sure the mount for the antenna is connected to the mast with a good electrical bond. You need to use an anti-oxidant like NOALOX on any connection to the aluminum mast. If there are any coax connectors as part of the run be sure they are sealed against moisture intrusion and that they are protected from direct exposure to the sun's UV rays.
Is your mast connected in any way to a connection that makes contact with the water? If not then what I would do is connect the center conductor of the coax cable directly to the base of the mast and use the mast as the antenna. You can use a short piece of copper wire to make the connection. Make sure it fits into the center socket of the radios antenna plug and is not making contact with the outer part of the connector. Attach the other end of the wire to the mast using an existing fastener. This is easy to test out without making any permanent connections. With an antenna connected to the radio and placed on the deck tune in a weak FM station. Now connect the center pin of the radio to the mast and see if the signal gets stronger. If it does then I would use this method. The fewer wires you have running to the top of the mast the better. If you go with coax cable, I would probably install one of these through deck waterproof fittings Sometimes
called marine glands. They are available as straight or 90 degree and allow cables to pass through decks and bulkheads with the connector attached.
They are available from many manufacturers but the advantage is that these is no break in the coax cable to allow water intrusion.
In my current job 90% of all unscheduled maintenance on cabling is due to water intrusion into connectors. Seems that no matter how good of a job I do sealing connectors the sun eventually degrades the sealant and lets water in.