Adult female elephants have two breasts, or mammary glands, located between their front legs. When a female becomes pregnant or is nursing her young, her mammary glands become more prominent.
In mammals that live in the water the nippels are normally protected with a skin flap. The infant needs to push this flap aside in order to drink the milk from its mother. The flap closes to be water tight, to protect the nipples.
For species that live in both the water and on land, it can be different. Sealions for example have retractable nipples. Just something to think about I guess.