Best is to use a good quality compression connector such as LRC snap n seal.
If these connectors are installed properly, the only target for water
intrusion is the threads. If you put a small dab of silicone grease on the
threads of the female portion of the connection before screwing on the
connector (make sure it's on the threads, not the center conductor), no
other water proofing is needed. We did an experiment at the cable company I
worked at when snap n seal connectors were first introduced. We sealed all
four connectors on a tap as above and put the tap in the bottom of a trash
barrell under three feet of water. After three months under water, there was
no sign of water intrusion.