If you're talking about a manicure, then yes, press on nails will come off in water. The adhesive that is used to apply the nails is not waterproof, so it will dissolve and the nails will fall off. If you're talking about false nails that have been glued on, then they may or may not come off in water. It depends on the type of glue that was used and how well it adhered the nail to the natural nail.