It seems silly to me to weld a beach cart to a 4-wheeler. If you're going to go through that much trouble, why not just weld a hitch to the ATV and tow it? A hitch will keep the ATV dry, you can still use it as an ATV, and you have less weight to move (ski and cart motor seperately) if the ATV isn't working.
you are not allowed to ride an atv on most beaches