Walkers do a good leave-in conditioner with sunscreen, which Hair Direct carry. But there are various ordinary supermarket brands which I'm sure work just as well.
My experience is that you can stay all day in the Mediterranean sun if you use a good dose of sunscreen and you top it up if you go in the sea, and there is no noticeable deterioration. But a whole week of sea and sun even with sunscreen would start to bleach the hair, so I would either get a straw hat or wear an old disposable piece.