Firstly, welcome to the Corfu Travel Guide, we hope enjoy searching through the many pages of information to help you further enjoy your Corfu Holiday.
The weather in May can be a mixture of weathers. Some times it has rained, and some times it has been a scorcher. Invariably it is very warm and sunny during the day, but can still get chilly through out the evenings, therefore I would suggest a jumper or cardigan for the nights out. The sea is swimmable, but can be somewhat, obviously, more colder than the summer months. Temps last year ranged from 18c to 26c thereabouts, sunshine from 1 - 14 hours per day throughout the month, with about 4 or 5 days rain throughout the whole month.
See the following link for a regular update on the weather in Corfu.