I'm getting married in England next May. My fiancée is from England. I am in charge of planning the honeymoon. She has hinted that she wants to go to Santorini. Does anyone have any thoughts regarding Santorini in early/mid-May? Any thoughts, ideas or advice would be very much appreciated!

Alternatively, any ideas on other places for a honeymoon in and around Europe? The only prerequisites are that it needs to be warm and on the water.

I have sailed in this area in early May.

It can be fairly warm, it can be quite cold with. You need go in Mid June to be sure of good weather.

Few European locations can be sure of warmth this early in the year.

Santorini is the quintesential honeymoon paradise. There will be lots of honeymooners there next May. The main villages are Fira and Oia which have views over the Caldera. They can be busy because of the cruise ships that visit daily so you might want to look at Firaestofani which is adjacent to Fira but the cruisers don't tend to wander that far. Although Santorini is expensive this is the place to stretch your budget to get that magic Caldera view hotel.
Santorini Beaches are pumice pebbles so it is not really a beach destination. If beaches are important you might want to consider planning a two island get away. Spend a few days on Santorini ( as long as your budget holds out) then hop a ferry and go to a companion island like Naxos where hotels on the ocean are inexpensive as is dining and touring around. There are other islands like Ios for the party scene or Paros which is similar to Naxos that could be a good second island.
Here are some images of Santorini
Santorini http://www.flickr.com/photos/stanbr5...7632154800782/
and Naxos in cast that might interest you.
Trip around Naxos http://www.flickr.com/photos/stanbr5...7634605629689/

Ah I forgot. Dickie is correct the later in May or early June the better the chances for good weather. This year it was windy in May but that wouldn't stop you from enjoying the sun and the scenery.