Halacha stipulates that mikveh immersion should be witnessed to ensure that all hair is submerged. This is the primary function of the mikveh attendant (balanit). Any halachically observant Jewish female over the age of twelve can fill this role. In pressing cases, in which no halachically observant woman is available, a couple should consult a halachic authority about how to proceed.
The mikveh attendant has several secondary roles. Her presence is a good safety measure against accidental drowning. She may also offer to check the hands, feet, and back for chatzitzot (barriers), and may quickly review the stages of preparation to ensure that nothing was forgotten. However, each woman is ultimately responsible for her own preparation and inspection (chafifah and iyun).
If no one is available to supervise immersion, a rabbi should be consulted for alternative solutions. A woman who is particularly embarrassed to be seen naked, even by another woman, should also consult a rabbi or a yoetzet to discuss how to use the mikveh in a way that will make her most comfortable.
Please see here for a more detailed discussion of the role of the mikveh attendant, and the question of immersion without an attendant.