Drain the water out of the cooler over night to dry, is this a standard cooler or a single air inlet? Some of the single inlet coolers have a plaste pan in them, if it does use RTV on the out side of the pan... That mines you will have toremove the pan to fix it.. If it is a standard cooler you can use mastic on it, but it has to be dry on the inside, use a small amount on inside, then on the bottom of the outside you can use more to patch it.. This way you won't have the smell of the mastic in your house, this will last for the season... I learned this from a kid that does coolers for a living, i got 2 yrs out of mineafter repair was done