What your sales rep probably was referring to was the "newer" rigs will not be damaged by the mismatched antenna. Most of the available antennas are plug and play and even if they have an adjustment, the difference in performance will probably be undetected without test gear. In other words, you are sliding the resonant frequency up or down to some portion or channel in the band. Some people will want it optimized on Ch16, but the noticeable difference to any other channel will be scant. If you are determined to have it 'perfect', you could bring it to a radio shop and have it tuned. It won't be worth what they charge. Or, if you know any amateur radio operators ( ham radio), they should be able to help you and can be paid in beer. No self-respecting ham will turn down free beer to work on radios. If you were in this area, I would check it out for free - It's what I do for fun.
There is another method that might work if it isn't much trouble adjusting the antenna. You need to receive a transmission from a not so near station that isn't moving. Some radios receive wx band. You could tune one of those and adjust the antenna to give you the loudest signal or most deflection on the signal meter if your rig has a meter. The problem here is that wx is around 162 MHz and most of the VHF Marine channels are 156-157MHz. I've done this with HF antennas and had OK results. It would be more difficult on VHF and probably not worth the trouble.
From what I've seen, the most common fault in antenna systems aren't the antennas themselves, but the transmission line and connectors. Replacing that and sealing the exposed connectors could make a huge difference. You don't need expensive coax for runs under 100' long at these frequencies. I like LMR 240. It's small (.240"), and has better performance than the typical RG8X or RG58 that will be sold with most marine vhf antennas. It costs around $0.79 per foot which could add up. There is no such thing as weatherproof or waterproof rf connectors. all should be sealed with something a little more durable than electrical tape alone. Liquid tape works ok, but is messy. Butyl tape is still the industry standard used by the pro's. the method used mostly is to attach the cable to the antenna with a properly installed connector ( crimp or solder isn't important as long as it's done right), wrap with electrical tape, wrap with butyl tape - not a ton of it, just enough to seal. Then wrap the whole mess in more electrical tape. If you replace your connectors, cut the coax back and if it isn't shiny copper, water has penetrated. You can try to cut it further back until you find clean shiny copper, or it's time to replace the cable.
