In Shei-gen, there is a hotel locationed on the west side of the Island, called The Daisy Chain. If you enter Shei-gen from the Los Muertos side, it is between the two highways. At the top of this hotel is a pool with a rubber duck toy. You can throw the toy at people to make a squeaky sound like a real rubber duck toy.
There is a rubber duck in the hot tub in La Mugre, which is the first island with Los Muertos. It is located at the apartment complex where you kill Violeta "Santa Maria" Sanchez.