Battery Tender Jr. Have two skis and one on each. Suggest you pull the battery so it doesnt freeze over the winter. I pull mine and put them in the garage, then will put the chargers on them in the garage.
After doing some research, I bought a NOCO Genius for 30 bucks at Amazon. Happy with it thus far, but only been using it one day. Last year, my traditional charger overcharged my battery and boiled it. Instead of buying a smart trickle charger for each of my two skis, I'm just going to move it back and forth every month or so over the winter. Make sure that whatever you buy is smart enough to turn totally off when it's not needed.
Don't forget to uncover and take the seats off for a few hours every week or two thru the winter to let the hull air-out! Don't leave it covered and buttoned up and forget about it all winter long.... :-(
Battery Tender Jr's are great, and you can get them delivered for around $26 off Ebay. But, whatever floats your boat.
If leaving your Battery or Batterys outside, dont leave them on the cement. I read a few writeup to place on a Rubber Mat, Let me try and fetch the Info.