Map from Google - Ibiza

The climate of Ibiza, an island of the Balearics, is typically Mediterranean, with mild and relatively rainy winters, and hot and sunny summers, a bit sultry but tempered by sea breezes.
The average temperature ranges from 11.5 °C (52.5 °F) in January and February, to 24.5 °C (76 °F) in August.
Here are the average temperatures.
Average temperatures - Ibiza
IbizaJanFebMarAprMayJunJulAugSepOctNovDec
IbizaJFMAMJJASOND
Min (°C)88910131720211915119
Max (°C)151517182226292927231916
Min (°F)464648505563687066595248
Max (°F)595963647279848481736661

Rainfall is not abundant, since it amounts to 450 millimetres (17.7 inches) per year, and follows the Mediterranean pattern, so it is more common in autumn and winter, and gradually decreases during spring, reaching a minimum in summer, when it almost never rains.
Here is the average precipitation.
Average precipitation - Ibiza
IbizaJanFebMarAprMayJunJulAugSepOctNovDecYear
IbizaJFMAMJJASONDY
Prec. (mm)40304040251552545705560450
Prec. (in)1.61.21.61.610.60.211.82.82.22.417.7
Days65665313456656

The Balearic Islands are pretty windy, especially from October to April, but Ibiza is the most sheltered from the north wind that blows from France, being the westernmost one, together with the nearby small island of Formentera. In summer, sea breezes blow, but they are welcome because they make the heat more bearable.

Winter, from December to February, is mild, with many sunny days, but also with periods of rain and wind. Cold days are rare, and in any case the temperature never drops below freezing (0 °C or 32 °F).

Summer, from June to mid-September, is typically Mediterranean, ie hot and sunny.

The amount of sunshine in Ibiza is not very good in winter, when Atlantic or Mediterranean depressions can pass, however, there's no shortage of sunny days also in this period. The sunniest period is from May to August, when clear skies are the norm. In September, the first Atlantic disturbances occur, so in autumn the sun does not always shine.

Ibiza

The sea is warm enough for swimming from July to September, while in June and October it's cool.
Here are the average sea temperatures.
Sea temperature - Ibiza
IbizaJanFebMarAprMayJunJulAugSepOctNovDec
IbizaJFMAMJJASOND
Sea (°C)151415161821242624221916
Sea (°F)595759616470757975726661

When to go

The best time to visit Ibiza for a beach holiday is summer, from June to mid-September. For swimming, the best period goes from July to September, although in September, which overall is still a good month (especially in the first half), the first rainy days may occur. May and June are sunny months, but the sea is still a bit cool, especially in May, when it is even a little cold.

April and May are suitable to visit the island and its cities. In October and November the temperatures are mild, but the days are shorter and the weather can be rainy and windy.