Wire is not just wire, but I said "existing COAX". Coax is coax and different types will all work to some degree. You'll get more or less loss with different types (say RG58 vs RG8X), but many boats' radios are rigged with the smaller, higher loss coax.
Marine VHF radios are not made to match any particular length or kind of coax. Marine VHF antennas are tuned with a loading coil in the base of the antenna, not in the radio or in a separate tuner (as in a backstay SSB antenna). Perhaps you're thinking of CB antennas, where the coax IS part of the tuned system and shouldn't be cut or changed.
The length of the coax to a 30 foot tower shouldn't be a problem either. In fact, its probably similar in length to a masthead antenna cable.
As Stefan pointed out, you'll have to square away your licensing issues with the local authorities if you plan to transmit!