Many wireless PCMCIA cards have a (very small!) jack for an external
antenna. There is really no practical and effective way to add an
antenna to an internal wireless card

Entirely not true. Most internal cards have a connector that wires them up
to an internal antenna. It's a matter of splicing in a new connector to
allow using an external cable. If you're handy with a soldering iron and
wiring then it's really not that difficult.

It may, however, be better to use a card that already supports one.

OR use a wireless bridge that lets you repeat the detected signal to one
your built-in card can use.

- you are going to have to spring
for a new card. Of course you will be looking for one with an antenna
jack, but also pay attention to the emitted power from the card. These
apparently come either as 20 milliwatts or 300 milliwatts. More is
better if your problem is getting the base station to hear your signal.

When you buy an external antenna, get one which is either two or 3
stacked verticals (two or three straight sections of wire, separated by
coiled sections). Then insert this antenna at the focal point of a
parabolic reflector: http://www.freeantennas.com/projects/template/

This combination will get you gain into the 20db range.

Which may be entirely illegal according to the FCC limits. While this may
not bother some folks be aware that the regs don't allow you to just tack on
different antennas without the *whole setup* having been approved for it.

That and using a parabolic antenna on a boat that's moving might be a bit
pointless (pun intended!).