Wifi is a two way street. The sum of transmit power and receive sensitivity for both ends have to be balanced. If you want to connect to a typical low power
(60-65mw) domestic access point at some distance you have to have very good sensitivity but receive sensitivity is usually the problem. It is fairly easy to add power but once you get past about 93db sensitivity starts getting expensive.
USB is fine for a portable unit but the USB signal is limited to 15' unless you put an extender at each end. USB extenders do work. I have one running to the far end of my house to operate a pan/tilt camera
mount but you add more up weight and may have problems supplying power to the remote
extender unless the device at the far end has its own power supply.
That is why the Ethernet with POE to a remote
router/bridge with a very short coax connection to a good 9db antenna is by far the best choice for a permanent installation. You don't have to worry about the length of an Ethernet cable on a boat unless maybe on Larry Ellison's Rising Sun and he probably gets a good discount and first rate installation support. :-)
up and configuring a bridge at the masthead to a single
PC is pretty straight forward (well except maybe for the mast
climbing part) but connecting it to a Wifi access point for a local wireless network adds to the complexity.
Incidentally, 9db is about as high as you should go for an antenna on a mono because any narrower beam width cause fading as the boat rolls. A cat can go up to 12db.