If there is a wi-fi point in or near the marina (or you can bring your laptop to it) all you need is a laptop with wifi. Newer ones come with it installed, but you can buy a USB wifi connector if necessary. I have found that it is best to let the software work automatically, rather than try to twiddle with the settings.
If the wi-fi point is not free you will oviously have to pay. The methods should be clear enough. Some of the wi-fi points will allow you to log-in enough to pay on line.
If you want or need to receive wi-fi over a longer range you will need some form of exernal antenna. In general, because of the high frequencies involved, it is not very practical to have an antenna cable longer than a few centimetres. I have a directional antenna with a wi-fi wireless inside the antenna box. It connects to my PC via an ethernet cable. A range of 500m should be easily achieved.
If you want more info please describe what you want to achieve - to avoid recommending unnecessarily expensive solutions.
I've got an old, well 6 years old, Sony Vaio laptop. I bought a belkin WiFi 'card' for it, and this summer(?) was able to connect easily via WiFi links at Premier and MDL marinas with no problems.
When you connect, you are prompted to pay for your WiFi hookup, usually about £5 per 24 hour period, or if you choose a week, I think that's about a tenner.
Simple and easy. In fact a damn sight easer than linking my laptop to my home WiFi hub /forums/images/graemlins/confused.gif
I have similar good results at Premier. With a USB dongle attached high up on the hull I get significantly better signal level than a laptop using built-in wifi.
You'd probably have more success using something with better range than a simple dongle. I use a Belkin G+ MIMO card, which is supposed to have excellent range, and I find it's only just about able to connect in many marina situations. Belkin also do a G+ MIMO USB adapter - like the card, it's about £40.
You don't say how far away you are from the wifi "transmitter".
I have one of these. Actually, I think mine is an older version. This is a directional antenna with a wifi radio inside the box - so you don't need a USB wifi device with it. You can power it from 12v or 24v (it works up to 48v, I believe). I have not needed to put mine outside - it works fine over about 500m from inside my boat. It is connecting to a regular indoor household wifi unit.
You can buy an extension lead for a usb connection - I have a couple that are about 3m long. You could use one or two of them to put a USB wifi device higher up. I can't advise whether it is worth buying a USB device and trying that before before buying the more expensive directional antenna.