Mark, if you mean temperatures, wind directions, wind forces month by month for a particular sea area then the Admiralty pilots contain it. More precise met. stuff I'm sure others will lead you to (ie I don't know!)
There may be some data on Martin Stubbs' site (www.weather.org.uk), but I doubt the kind of thing you are looking for exists on the Net. The Met Office Weather library (located in its headquarters at Bracknell) holds this data and (I believe) is accessible by the public. Depending on what you are looking for, Alan Watts published a book (Wind Pilot) a few years back which analysed wind speeds and direction for a number of sites around the UK and Europe. Its out of print now, but available in second hand shops (some are on the web)
It IS available commercially, in computer readable form(CD) with a hardback book, unfortunately it covers the world. The price I believe is 950 (I dont recall whether it was £ or $, it was so high it was immatrial!)