The average sea temperature is still below 20°C, which may be too cold for many people. Can you swim in Tenerife in April? Yes, but it depends on the beach and each person's preference in regards to sea temperature.
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Lanzarote and Fuerteventura are the hottest islands in April, though Gran Canaria remains warmer at night. Fuerteventura is the driest, with only 5mm of rain predicted for the month.
Of course, on the Mediterranean coast and the further south you go, the heat will rise, which means the sun will be out, and you can start sunbathing on Spain's beautiful beaches and its offshore islands. Alas, in most places in Spain, the water will still be too chilly for swimming.