These two words get used interchangeably in cooler bag listings and they describe two genuinely different products.

Water resistant

A coated liner, stitched in. It shrugs off condensation from a chilled bottle and copes with a small spill.

It has stitch holes below the fill line, so a bag of melted ice will find them eventually. That is not a defect; it is the construction.

Leakproof

A welded, seamless liner with no stitching at all below the fill line. It holds melted ice, and it rinses out at the end of the day.

It costs more and it is worth it wherever melt is guaranteed: a full day out, catering transport, fishing, anything travelling in a vehicle.

How to tell which you have been sent

Turn the bag inside out, or look into the corners with a torch. Visible stitching inside the liner means water resistant.

A welded liner has smooth radiused corners and no thread anywhere below the fill line.

When not to pay for it

Lunch bags, short trips, supermarket runs. There is no melt to hold and the money does more on the print or on the zip flap.

We will say so rather than upselling a capability nobody in your programme will use.