Ewan,
There is a bunch of links on the Fish Head site related to this, though it is only pertinent to the Australian coastline with a bias towards the East coast.
What locales are you looking for data on?
Most of the easier non-pay-per-view sites for Oz are on the Fish Head site.
seasurface.com and there was another I think called geoeye that did larger areas on sea surface and phytoplankton readings, though the costs start at around $180 AUD for a yearly subscription and go up depending on the amount of services you dip into.
I have used some of a mates print-outs off the seasurface.com site when marlin and general pelagic fishing and it has been good.
There is certainly several levels of info better again depending on your wallet.
It may seem dear, though the few times I have had a crack at using it for blue marlin off Brisbane, it has worked out in finding fish every time!
I am telling my other half to consider this for my Xmas present, it helps that much.
Cheaper than burning fuel to a spot washed in green pea soup!
Cheers
Aaron.