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I'm not as familiar with marine grade coax. That being said, yes coax degrades with age, some slower than others. On one hand if it ain't broke don't fix it sounds good. On the other hand if you may need to rely on the VHF for safety replacing it could be a good idea.
Use high quality cable like btrayfors suggested, and pay attention to the end connectors. I use a 100 watt soldering iron that has a lot of mass to solder them on. Don't use the pistol shaped sldering guns, they get too hot and don't have enough mass to transfer the heat effectively. Seal the mast head end with proper coax sealer. If your library has it, you can look at the ARRL Antenna Book for some good info.
I wouldn't worry about thick, low loss cable. Just use a high quality coax that fits. Connecting and sealing the ends is extremely important. Pay attention to what the connectors are. PL259 most likely, but N connectors are much more waterproof and it is possible to have these on the outside run.