Merpeople may have slightly enhanced senses suited to life underwater due to living in the sea. To sea born mermaids, land people smell horrible, even the average hygienic human emits a smell that only mermaids can detect. However, they can get used to the smell. By smelling Rita's supply of squid ink, Lyla could identify where the species of squid came from.
In H2O: Mermaid Adventures, mermaids have the same range of hearing as dolphins. With this they can hear things humans cannot but their human half allows them to hear sounds cetaceans can't.