On many of the flat and wide beaches (the most obvious example being Sotavento in Jandia) you will notice big differences between high and low tide. They generally follow approximately in 6 hour intervals, with every day having 2x high tide and 2x low tide. The times of day and even the water height vary. The best way to know is to check the current tide forecast for the days of your stay. The following are a few websites where you can do that.
Fuerteventura Playas – with detailed times and water height predictions for every day the current month and the next month
Magic Sea Weed – with detailed tide times, height and other details specifically for surfing in several of the popular surfing spots around Fuerteventura, including Cotillo, La Pared, Las Salinas, the east coast, the shipwreck (near Pajara / Ajuy) and Cruz Roja (Morro Jable).
Tides4fishing – with detailed tide times and other useful information specifically for fishing, available for two locations on Fuerteventura: Puerto del Rosario and Morro Jable