I bought flexible plastic conduit and plastic fittings at Lowes. Worked a charm. Through deck
fittings are the type that fit into the junction box with a hose barb to slip the conduit over. It has been on the boat for a year now with no sign of UV damage. Price
is right so will just buy new fittings and replace if UV damage becomes an Issue.
Have used the above pictured through deck fittings in other applications and they have been fine with a single
wire like a coax cable. Imagine you'd have to goop these up with sealant
to get them to seal with a two lead. The Blue Sea cable clam fittings are great if you have a large end fitting on the wire like a coax connector. You drill a hole the diameter of the wire in the center rubber part. It's already split so you just wrap the rubber donut around the wire and tighten it up.
These fittings are a bit pricey, however.
The metal fittings will only let the wire through the fitting so either have to cut off or not have installed larger end fittings. AS with the above, you'll have to goop up a two wire lead to get them to seal.