You can find different kind of beaches in the islands for every taste.
The north/east part of the Islands usually has strong currents and waves although you can find some naturals pools or other natural formations that make the waters safer. Furthermore, most touristic places have some breakwaters (barriers to break the waves and divert the currents) so they are safe
On the other hand, the south/west of the islands is usually calmer but It is also the most touristic part of the islands. Also the weather is usually better and there are some beaches with breakwaters.
If you are Visiting Fuerteventura or Lanzarote, I recommend you the west part of the island since it is usually calmer sea but try to avoid beaches where people practice water sports such us windsurf or kite surf since they are usually windy.
I remember some beaches in Fuerteventura:
- Gran Tarajal is a wornderful beach not very touristic, clean waters and usually calm waters.
- Caleta de Fuste has a water barrier so it is safe any time of the year and it has a lot of ammenities around.
- Playa del Matorral It is a big beach and due to the shape of the coast, It has to different currents in the beach so if one part is not calm the other one usually is (This is also applicable to the wind). Furthermore, This beach is usually less windy than others in the island.
If you are not interested in renting a car Morrojable (Playa del Matorral) would be your best option in Fuerteventura. If you have a car, you can move to one side of the island to the opposite.
Note: I usually plan my trips to Canary Islands with ResidenteCanario since it covers more than any other page but It is a Spanish page so you would need some tool for translating the content.