I've wired up 27Mhz antennas in the past and left the co-ax conections exposed or just covered with a rubber boot but self amalgamating tape, silicone sealant and a few other things will seal it or help to keep corrosion at bay adequately if you are that worried.
Get the mount on, get the SWR as low as poss and Bob is your long lost uncle who is getting slightly concerned that you may be worrying unduly about waterproofing. Keep a regular check on the SWR and the cleanliness of the connections