If you want to take your antenna cable through a bulkhead, or cabin top, or deck for that matter, one choice is the PL363 connector with two PL259 plugs.
The one in the picture is a 1.75” connector fitted with two large stainless nuts. These connectors come as standard with rather narrow-walled nuts, quite useless, particularly if you want to use sealant at the joint. So, add the hefty large diameter nuts.
To take cables through a deck you may want to look at the Cableport, a wonderful Swedish design that’s standard equipment on many top quality boats. Check it out on the Salty John website.
Other choices would be deck plugs, deck glands or a swan neck. Deck plugs are notoriously corrosion prone and need meticulous maintenance. There’s a wide range of deck glands available; you’ll probably want the type through which you can pass a 19mm diameter PL259 connector, especially if you drop the mast each season.
Good quality connections minimise losses in the antenna system.