The antenna transmission line shown in Figures 1 and 2 from a 1996 boat is probably from that same epoch, and is thus approaching 25-years or service.
Because the portion of the transmission line from the base of the antenna to the the pass-through fitting on the gunwale is subject to flexing when the antenna ratchet base mount is moved to lower the antenna, the motion has caused cracking of the insulation when the insulation lost its original flexibility. Also, there was probably insufficient slack in the cable so the cable was flexed too much when the antenna was lowered.
TEMPORARY REPAIR To make a very temporary repair, I suggest using 3M SCOTCH 35 Vinyl Electrical tape in the white color. This tape is sold in a width of 3/4-inch. To create a flexible repair, unroll a suitable length of the tape. Using a sharp knife (an Exacto blade is highly recommended) cut the tape width down to about 1/4-inch. Apply the tape in 1/4-inch-wide segments, not in a spiral winding, with a thickness of not more than two raps. Use many short individual windings. This will create some protection for the transmission line but allow it to remain flexible. If there are areas where all the original insulation is removed, apply tape to those areas first, then re-cover with a second layer when repairing the whole length.
I do NOT recommend trying to make a repair of the cable as described and illustrated above with various splice or barrel connectors. The deterioration of the original transmission line is too great to permit such devices to be used effectively. And adding unnecessary connectors to the transmission only increases chances for failures and problems.
PERMANENT REPLACEMENTBecause I consider having a 25-watt fixed-mount VHF Marine Band radio with a proper fixed antenna as essential safety gear, I recommend for the long term solution to this problem the entire transmission line length should be replaced. However, since for many VHF Marine Band antennas the transmission line is integral with the antenna, this may mean you will be replacing the antenna.
A service life of 25-years for a VHF Marine Band fiberglass antenna is outstanding, so I would not fret about replacement of it. The cause of the problem is likely due to insufficient slack in the transmission line between the antenna base and the gunwale fitting. When installing a new antenna, leave more slack at this point.
COAXIAL TRANSMISSION LINE OUTER JACKETThe outer vinyl insulation jacket on coaxial transmission lines that will be used outdoors MUST be made from a compound generally described in radio transmission line literature as "Type-II Non-contaminating vinyl jacket." This type outer jacket for coaxial transmission lines is generally also resistant to decay from UV.
In the marine electronics business there are many coaxial transmission line cables sold under the description of "marine grade", but there really is not a particularly recognized standard for a "marine grade" cable, and this term is often used just to indicate a cable with a white vinyl jacket.
When choosing a new antenna, look for a manufacturer that specifies the transmission line provided will be use a Type-II Non-contaminating vinyl outer jacket.
CAPTIVATING TRANSMISSION LINEWhen an antenna transmission line is passed through a small gland or fitting in a bulkhead, the opening is generally too small to allow a connector to be passed through. (In electronic installation, we say the cable with connectors has been captivated by the fitting.) This means that the connector for the transmission line at the radio must be installed after the cable has passed through the fitting. This means installation must be done on the boat. Or, for removal of the cable, the connector must be cut off. For some advice on how to avoid this problem, see my article on this topic at
A Captivating Saga of VHF Antennasviewtopic.php?f=13&t=5734The above article suggest use of a particular bulkhead fitting or gland (Blue Sea Systems) and a particular type of very small diameter connector (the FME female).
I also note that in the installation shown in Figure 2 above, the antenna cannot be unthreaded from the mount until the transmission line is snaked back from the radio, and its connector cut off. This makes servicing the antenna quite difficult.