There are several places water can get into any given jack. Water should be able to drain out though. If the exit points which would be at the lowest point are clogged then the water would not be able to exit. You could use a jack cover like # UF38-944020 for future use. There is no telling how much damage is really done. If there is enough corrosion then something inside could be structurally compromised and fail when you have weight on the jack. I would not be comfortable offering...
One pretty awesome solution is the Towing Organizer by GR Innovations. This made in the USA organizational solution attaches around your trailer jack (either manual or electric) and gives you a place to hang two safety chains, an emergency brake cable, and an electrical harness. Making everything neat and organized (and making Marshall a very happy camper).
The efficient ball screw on this powered jack ensures smooth, quiet operation. Easy-to-access toggle switch and bright, side-mounted LEDs make setup a cinch. Durable, corrosion-resistant jack offers Features: Electric jack with 12V motor lets you lift and lower your A-frame trailer quickly and easily Efficient ball screw makes it fast and easy to position your trailer Ball bearings roll along
Use this electric tongue jack to lift your trailer with the flip of a switch. The sturdy steel jack can handle up to 3,500 lbs. It has 19" of travel and a 5" drop leg for extra lift. LED Features: Electric jack with 12V motor lets you lift and lower your A-frame trailer quickly and easily Top-mounted toggle switches are easily accessible Large LED side lights make it simple to set
Bought one to protect the head unit on boat trailer electric jack. Works great. Bought two more to protect our eBike (pedelec) computers on the handlebars of our bicycles. Fits nicely over the brake handles and the computers to protect them from inclement weather. Our bikes sit on an external bike rack when we travel. I had to take apart the computers and use a hairdryer on them after out last two thousand mile trip. Hope these eliminate having to do that and prevent future damage to the computers. 319085
Keep your trailer in line with this system that uses a telescoping bar with friction pads to correct trailer sway. Built-in grease zerks in the head provide simple maintenance. Includes an adjustable maintenance Powder coated steel resists corrosion Snap-up lever eases hookup by reducing lifting required by trailer jack System includes head assembly, adjustable shank, 2-5/16" hitch ball