Soto's Lightweight Spackle is not waterproof and should not be used on things that are submerged underwater. If it receives a waterproof coating on top of it (for example, if you paint over it), it can withstand some exposure to water, such as being cleaned or rained on.
If you need to fill in a place exposed to frequent water, then our Appliance + Porcelain 8-piece Epoxy Repair Kit may be needed to fill in the space.