When it comes to putting tires on a dog getter go with a full size truck tire, maybe some old mud tires or some thing that are wide. It floats alot better... more surface area to spread the weight over. Those boat tires do not make it around a wet corn field.
Plus... if you ever just want to put it behind you truck you got some full size tires to run on. Spindles and hubs aren't that expensive and you can usually find some old Chevy 15' 5 on 5 wheels around.
This is my dog wagon. Floats right across the mud... don't even know its back there. I used this rig before the MULE.