You have several options - consider what you want to achieve.
The most limiting factor will be WiFi availability. You need to get on the internet, so the question is how to you do it from your location (wherever that is).
On land WiFi (I am not referring to dedicated satellite internet service, such as Hughes Net, Iridium, GlobalStar, etc, but instead referring to internet WiFi based upon POTS [Plain Old Telephone System]) is achieved via the telephone (land line phones) or cell phone repeaters located across the country (world). If you're in range of a free WiFi signal (such as a marina, another person's WiFi at their home, or a business (assuming they are open to Anonymous usage and not using security), you can "generally" get a signal. Howver, that signal (your ability to connect to the WiFi) will vary greatly based upon your location to the WiFi signal.
So, in some marinas or in some locations, you may or you may not get a very good signal - generally based upon your distance from the WiFi signal. Just remember, some marinas have great WiFi equipment and great amplification of the WiFi signal, while others you can't get a signal outside of 100 feet - it's hit or miss.
Some, cell phone providers/carriers (AT&T, Nextel, Verizon, etc, etc) have decent cell phone repeaters located along coast line areas (either freshwater areas or salt water coastlines). Again, it's hit or miss as to if they have a repeater near shore and someone (a business, personal home) has a WiFi set up near where you are located.
The above basically all refers to getting a signal on land. And when you're on a boat, land is going to be generally very limited on a boat - you will need sight of land to get a signal.
Sometimes, you can get a decent signal off-shore for a limited range via your cell phone - again, depends on the provider's/carrier's location and signal strength.
I have a BlackJack II SmartPhone and my carrier is AT&T on the 3G network.
When a land-line WiFi signal is not available or the location is security enabled (e.g. WEP), I hook up my phone and use my tethering. This equates to making a call on my cell phone and using the modem connection on my cell phone with my laptop. Around the area I'm located I can sometimes get 25 miles offshore - not bad!
Outside of the above, if you are really talking about offshore internet access, you're talking satellite internet systems which run on average of about $400-$500/month for roaming the seas and getting on the internet - and you'll need: (1) the equipment (expensive), (2) a minimum of a one year contract (expensive); and, mucho $$.
So, that wasn't very brief, but getting WiFi access on a boat is very, very limited to marinas (and the equipment they put up for you - will it be free, are you in 'range' of their WiFi), and you're limited to on land repeaters and equipment maintained in the area.
The other option, is to look into tethering as I do with my phone. It works most of the time, but again, I'm limited based upon whether the carriers in the area support the 3G network, and whether the carriers in the area perform roaming on other cell phone provider's/carrier's networks.
Think about where you will be most of the time in trying to get a WiFi signal.