Anyone have a link to an external marine Wifi antenna. I want to mount it
atop my mast and run about 40 ft to my computer.

A couple of points to consider. The farther you put the equipment from the
antenna the more signal you'll lose. It's often better to put the equipment
as close as possible to the antenna. Even higher quality cabling (don't
even bother with cheap stuff) will cause signal loss.

It's correct to state you'll need an omnidirectional antenna. The boat's
movement on anchor won't ever make it practical to try using a directional
antenna. You'd be constantly fidding with the antenna trying to keep it
aimed properly.

I'm sure there are more technically accurate terms to be applied here,
forgive me for using a layman's explanation.

That and mounting the antenna up high has it's own hassles. The signal
pattern from the antenna is doughnut-shaped. It tends to be a rather tight
vertical angle. So the closer you are to the target, the tighter the
vertical aperature will be. So if you're too close to the base you won't be
able to pick it up. But then there's the opposite side of the coin, the
wider aperature will end up picking up MANY more signals from father away.
This has the downside of reducing performance, as you end up 'hearing' too
many Wifi networks.

Then couple the aperature issues with a boat that's swinging on anchor. An
antenna mounted higher up will end up causing signal loss as the wave action
swings the aperature away. The donut keeps tilting up/down and 'misses'
the base station.

I used a 10db omni up on the radar arch. I ran the cabling to an access
point mounted inside the arch (cable was about 18" total). From there I ran
CAT5 to another access point below in the cabin. The one on the arch
connects to shore as a client. The one in the boat provides a network to
things just on board (I've got the power set quite low on it) Works great,
provided you can find reliable free WiFi.

And after two seasons of screwing around with trying to find reliable free
Wifi I decided to just use cell data instead. I picked up a Verizon data
card and a Linksys WRT54GS3G router for it. Works great, even just using
the plain data card antenna plugged into the router while it's below decks.

So before you go through all the effort of fiddling with wifi, with the
expectations of getting something "free", you should really look at whether
a cell data plan would provide effective coverage instead. For the
$50/month it costs for the card (which gets used in laptops during trips
otherwise) it's been well worth it for us.

-Bill Kearney