Do you really have to charge your battery so often that this is an issue? If battery dies, recharge (on lift is fine, just keep everything dry), check alt output. If alt is good and battery dies again, replace the battery. No worries. I normally only charge mine at the beginning of the season. If everything is working well and boat doesn't sit in the water leaking and constantly cycling bilge it should be rare that you ever have to charge it.
But yeah, just keep your charger and such dry and no problem charging on the lift. If you're really worried you could get one of those marine automagic trickle charger jobbies and install it in the boat so you can plug it in all the time to keep battery maintained (good if you rarely use the boat).