As for the water, I'm going to disagree with HydroGuy
I think the water will be just fine. My birthday lands in those dates. I have never thought twice about planning swim parties. Throughout my life (almost all of which I have lived in So. Cal.) I have had many pool / beach parties for my birthday.
Will the ocean be as warm as July? No, but we've always gone in the water - and you will have plenty of company. The following weekend is Memorial Day weekend when the beaches are filled with swimmers, surfers, etc....