Map from Google - Costa_Blanca

Costa Blanca ("White Coast"), the part of the Spanish southern coast belonging to the province of Alicante, has a Mediterranean climate, with mild and relatively rainy winters, and hot and sunny summers. Because of its southeast facing position, Costa Blanca has a mild and sunny microclimate. There are towns like Denia, Javea, Moraira, Calpe, Altea, Benidorm, Villajoyosa, El Campello, Alicante, Elche, Guardamar del Segura, Torrevieja.
The average temperature of January and February is about 12 °C (53.5 °F), that of July and August is about 26 °C (79 °F). There are many sunny days, in which the daytime temperature reaches or exceeds 20 °C (68 °F), even in winter. In winter, very cold days are rare, but on colder periods the minimum temperature can drop to about freezing; in practice it never snows. The coldest records are around -2/-3 °C (27/28 °F). In summer, the heat is generally bearable and sea breezes blow, but there may be periods of intense heat, in which the temperature can reach as high as 38/40 °C (100/104 °F).
Here are the average temperatures of Torrevieja.  
Average temperatures - Torrevieja
TorreviejaJanFebMarAprMayJunJulAugSepOctNovDec
TorreviejaJFMAMJJASOND
Min (°C)891113162023242117129
Max (°C)161618202326292927242017
Min (°F)464852556168737570635448
Max (°F)616164687379848481756863

The rains in Costa Blanca are scarce, being even lower than 300 millimetres (12 inches) per year, and follow the Mediterranean pattern, in fact most of them occur from September to March, while in summer it almost never rains. Rainy days are particularly rare, but when it rains, it can rain a lot, and sometimes the rains may be intense and concentrated in time, causing floods. Due to the scarcity of rainfall, the landscape is arid.
This is the average rainfall in Torrevieja.
Average precipitation - Torrevieja
TorreviejaJanFebMarAprMayJunJulAugSepOctNovDecYear
TorreviejaJFMAMJJASONDY
Prec. (mm)302520302082645353530280
Prec. (in)1.210.81.20.80.30.10.21.81.41.41.211
Days33333101334331

The amount of sunshine in Costa Blanca is decent in winter, when sometimes Atlantic or Mediterranean disturbances can pass, however there is a good number of sunny days (there are on average 6 hours of sunshine per day even from November to January, and 7 in February). Spring is a sunny season, especially in May; the sunniest period is from June to August, when clear skies are the norm. In September, the first Atlantic fronts begin to pass, so in autumn the sun is not always guaranteed.

Calpe

The sea in Costa Blanca is warm enough for swimming from July to September; in June it's still cool. Here, for example, the temperature of the sea at Benidorm.
Sea temperature - Benidorm
BenidormJanFebMarAprMayJunJulAugSepOctNovDec
BenidormJFMAMJJASOND
Sea (°C)141414151721242524211816
Sea (°F)575757596370757775706461

When to go

The best time for a beach holiday in Costa Blanca is the summer, from June to August. For swimming, the best time is from July to September, although in September, which overall is still a good month (especially in the first half), the first Atlantic fronts begin to show up. May and June are sunny months, but the sea is still a bit cool, especially in May, when it is actually a bit cold.
Even though good weather is not always guaranteed, spring and autumn are ideal for visiting cities and making excursions into the interior, as in summer it can be very hot.