Four runs of lamp cord would work to some degree too but CAT5 is built to strict standards. Just splicing it together will probably be a bit less than desirable. Spring for the crimper or pay a network nerd a couple bucks to crimp it for you.
I have an EOC-3220 EXT on the bench right now. Cleaning the corrosion off the board, repairing some traces and replacing the RJ-45 socket and a couple of SMD resistors. It had been mounted at the masthead of a friends catamaran in the BVI and wired with regular CAT5. The jacket wore through as it entered the mast about 6" below the box and water wicked up the wires and into the case. It worked fine until the 48V lines started arcing between contacts. Heading for Tortola this weekend to reinstall it. OTOH, if he had used gel filled I wouldn't have had an excuse to go sailing.
Cantennas are directional. They would be fine if you are tied to a slip and know exactly where the access point antenna is but if you are that close you probably don't need it. At anchor or on a mooring and going for distance you will loose the signal every time the wind or tide shifted. A 9db non directional antenna would be the optimum. Any higher and you would loose signal as the boat heels more than about 10-12 degrees.
The antenna that comes with these units have 6db gain. The 9db doubles the effective radiated output but may not quite double the usable range.