I am about a 10 minute drive from Guardamar,I would say your weather, forcast is likely to be pretty accurate,but the lovely weather does dip suddenly when the sun goes down so perhaps a couple of jumpers may be the way to go.
The Valencian web site,gives live camera views of the beach at Guardamar,so you can see just how nice it looks in sunshine hours
Was there a couple of days ago,and a few hardy types were in the sea.,