Hi and welcome to the forum
The weather in mid May is likely to be warm and sunny by day but the evenings could be cool enough to warrant a thin jumper or cardigan if you are out and about.
It
could rain but it's unlikely and then it'll only be a shower, not enough to need a raincoat. It's a very pleasant (and increasingly popular) time of year to visit with plenty of warmth without the oppressive heat of high summer. Temperatures in the mid to high twenties. I'd suggest you just pack for a summer holiday (shorts, t-shirts, that kind of thing) with thin jumpers for the evening.
You can, of course, swim in the sea at any time of year but it's likely to be cool in May. Not cold, it's never cold, at least by UK standards, but a little bit
bracing