I own a Toyota Prius which uses the smart keys, and I have rented many other vehicles that also use the smart feature. There is a single door key hidden inside the smart key, that is supposed to be used if for some reason the smart lock doesn't work, like if the battery in the key is dead. This small key is easily removed and stashed in a pocket, etc.
The other problem could be if they use the system like on the 2005 mustang. The key's have a microchip inside the key. You can get a copy made of the key at homedepot, but the copy wont work due to the missing chip. I dont know how these keys will do submerged in water............