The Canary islands have a moderate, subtropical, climate. In July in coastal areas you can expect temperatures in the 20-35º range, with the average around 25º. Humidity in Las Palmas city is typically high and will often be around the 70% range, but higher and lower values are possible. Expect no rain whatsoever. By the sea, the cool breeze does make things more comfortable so you shouldn't feel too hot.
In practice, by day you'll be most comfortable wearing short-sleeve t-shirts and shorts, summer-weight trousers or skirts. In the evening it will cool a bit and a light jacket or long-sleeved shirt is suggested for all but those like it cool.
The local beach, Las Canteras, is used for swimming and sun-bathing year-round. It's on the Atlantic and therefore slightly cooler than the Mediterranean, but comfortable for all. In practice the water may feel a bit cold if you've been in the sun for a while, but once you get in you should be able to stay in for as long you like.
The beach is safest for bathing in its nothern half (away from the Auditorium) - so average swimmers and children are advised to use that part. The area closest to the Auditorium has larger waves and is sometimes used by surfers. Some areas are also used for playing sports. If in doubt take a look around for signs and what the locals are doing.
Those who feel abit adventurous may want to swim to the reef (called "la barra") which runs parallel to the beach at low tide. It's a good place to dive and see the local fauna, or just take an unusual stroll.
The climate gets more extreme towards the mountains, so if going inland wear cooler clothing by day and bring a jacket and long trousers if staying into the evening.
Ocassionally hot, dusty air from the Sahara will come this way for a few days. But the air remains cool and clean by the sea. Inland, it will just feel hotter and drier.
Often in summer it'll be partly cloudy in Las Palmas - known locally as "donkey belly days". But generally it won't feel cooler and you can still get sunburnt if your skin is light-coloured. This is caused by the "Trade winds" stopping at the mountains. It will still be sunny in the southern side and top of the island.