there are a few things that can be done to water lines to protect them from freezing. my first thought is why is there a need for water on a dock during freezing temperatures. are we talking about the water lines that are used to supply water to the hose bibs that are used to wash down the boats? if so...is there a need for water during this time of year. if ther is no need for water on the docks during this time then simply install a shut off valve at the very beginning of the water line that supplies water to the hose bibs. you would shut the valve, open the hose bibs to drain the water out of the lines and then close them again. pipes cant freeze if there's no water in them. if there is a need for water during those freezing months then you would need to insulate the entire run of pipe. i would suggest double wrapping them and tape all joints. another way to prevent freezing is to slightly open the last hose bib on the line just enough to get a good drip. running water wont freeze. you can also use heat wire if your in temps that are in sub zero weather. its very simple to do, just coil the wire (loose coil) the pipe doesent have to be completely covered, tape it on so its secure and then insulate over that. plug in the wire and you should have no problems.