Are you cruising or on a residential mooring?
This can be the big drawback to a working life afloat so if you come up with a good answer let me know!
From what I have found to date:
1. If on a mooring and AT ALL POSSIBLE try and get your own fixed line or wireless to fixed line connection for both cost and simplicity.
2. If no fixed line but mainly on a regularly mooring near civilization, look at the possibility of wireless hotspots with an aerial booster (cantenna). Sometimes even councils have wireless services they will allow you to access or aim for a McD's etc with a wireless hotspot.
3. For low usage (emails etc) look at access via the mobile phone network.
4. The most costly (but most 'take it wherever you go') must be two-way satellite broadband (note: one-way satellite also needs a phone line). Still, if you need it for business use to earn your living I'd guess any capital outlay and monthly subscription must be offsettable as a legitable business expense?
Have a look at sites like: http://www.fastnetbroadband.com/fastxone/index.html , http://www.bluestarinternet.net
and (especially for motorhomes/boats etc ?) possibly the ultimate
http://datastorm-mobile-satellite.co.uk/
Good luck and don't forget to let us all know if you find a good solution.
Steve, August 2007