Purchase 2) 4.80/4.00-8 Inner Tubes from Lowes for $10ea.
Either cut rubber tires off wheels or spin a wheel while on the cart and heat rubber with a hot air gun. Remove and using a bit of effort, pull the tire off.
There are some plastic thorns on the wheels that need to be ground off so they don't puncture the inner tubes. I used a wire wheel on my Drill to make quick work of this step.
Drill a hole on the wheel for the stem to slip through. I placed on the inside of the wheel. Orient the hole as shown between an opening in the wheel.
Partially inflate the inner tubes. Slip the stem through the hole and slide the rim on. Now inflate with very little air. Use just enough to hold up a loaded cart where the tire will sag to displace itself over the sand. That is the key to easy rolling.
That's it, head to the beach and enjoy how easy it is to get to the water.
Edited: the manufactured wheels only make contact with the axle towards the outside causing them to tilt in over time and under load so a nylon washer was added on the inside (last two photos) of each wheel to keep them spinning true, parallel, and much smoother. The washers ate the correct ID but the OD was .05 too big so I Turned them down. Used a drill bit with the diameter just slightly larger than the ID and it slid on tight. Place into Drill chuck and spin the washer against a razor blade using the edge of a 1x 2 as a support guide. Photo doesn't show nylon washer since It was already On the wheels when photo was taken. Keep the OD slightly oversize and tap in tight with a hammer.