Cable attenuation can be expressed as the watts transmitted into the cable and the watts radiated from the antenna. We could use dB loss which is based on a log scale, but lets not be complicated. We have two basic coaxial cables under consideration, RG58 with a diameter of 5 mm (.2 inch that looks about like a quarter inch in diameter but is actually a little smaller) and RG8 with a diameter of 10 mm (.4 inch). The installation is for a VHF boating frequencies so I used the 160 megahertz (MHz) frequency for calculations. Since the distance from top of mast to deck is 31.5 feet, I used 40 feet as the length of coax cable. At the VHF frequencies there is a lot of loss of power because of what are called surface effects and dialectic loss. There is a little less dialectic loss in cable that has a foam core. When transmitting at 25 watts using RG58 coax cable, 15 watts make it up to the antenna. With RG8 cable 20 watts make it to the antenna. Of course at 5 watts transmitting power the radiated power from the antenna is 4 and 3 watts for RG8 and RG58 coax. The impedance is 50 ohm. Do not use TV cabling, adaptors, antenna, and connectors which are 75 ohm.
Here is a web site for sealing coax connectors: Waterproofing a Coax connector splice
Here is one for putting the connector to the cable: How to install a coax connector- Boating and Fishing Forum
Here is the calculator for coax cable loss: Coax Loss Calculator