Plan on replacing the switch some day. And you should have access to it for testing when you get a system failure and need to troubleshoot it.
I would run a conduit to the tank and bring it above ground to a box that is designed for outside service. It need not be a large box. Then I would run conduit from that box to the controller. If you come into the box from the bottom, it dosen't have to be sealed perfectly. In fact, it is better to let it drain. It is almost impossible to seal a box perfectly and you will usually end up with condensation, and therefore 100% humidity, in a "sealed" but not really sealed box.
If forced to do an underground splice, I would use the sealing shrink tubing materials that are used for submersible pumps. They have a sealant inside the tubing. That means you have to dig a hole to repair it; which is not fun if you live where you get 4 ft of frozen ground.