Summertime brings a whole unique set of skin challenges, and the dry, cracked skin on your feet is not an unusual concern. I have two thoughts that could be the cause of these fissured feet.
First, summertime leads to wet, sometimes sweaty feet, and a lot of in and out of pools and oceans. Feet getting wet, and then rapidly drying, causes some dryness on your soles. Also, sun exposure can dry out the skin.
The second potential cause for dry, cracked skin is an issue with your flip flops. We see some patients that have allergies to rubber accelerants found in many flip flops. The starting point for both of those potential conditions is really, really thick hydrating moisturizers and keratolytics. Some of these are over the counter, like Salex and Lac-Hydrin Cream. These can be very effective in both hydrating the skin and also exfoliating some of the dead skin on the sole of your foot, which can return normal moisture to the skin faster.
The second preventative method is to make sure that anytime you get your feet wet or you have extensive sweating, gently rinse off your feet and apply a moisturize over top. One of my favorite moisturizers are ceramide-based moisturizers. Please look out for ceramide-based moisturizers instead of petrolatum-based ones, such as Vaseline, because the heat can sometimes make things worse.