Bagging the boots is a reasonable suggestion, but I'm concerned that if I forget to do it just one time, I may have contaminated my suit forever.
But that prevents you from doing the shore diver's trick that someone mentioned above of filling the fins with water on exit to use like buckets to rinse the dirt off when you reach your changing bench/mat.
Any McGyvers out there who can figure out how to put some kind of removable rubber plug/valve in the hole? Some kind of one-way plug or valve that breaks the vacuum but then seals again upon filling the foot pocket with water.
Just leave a gallon milk-jug full of water in the car. Fins are for diving, jugs are for transporting water. More water than you could possibly fit in your fins (if you're able to get them off).