Santa Cruz de Tenerife's only city beach is between the city port and the cargo shipyard. However, this is an acceptable option if you are looking for a quick refreshment. There is clean water, fresh air, but there are no beach facilities, so you need to bring your own towel and drinks.
This is a good place for a quick swim in the heat or a picnic at sunset.
However, if you want the perfect oceanfront getaway, hop on bus number 910, go to Teresitas Beach, and enjoy all sorts of beach fun.