Swimming Without A Net by Mary Janice Davidson.
*again with this is not suitable for children/young teens due to some of the content*
In this second book in the series Fred is trying to fit in with her own kind, she finds she's hooked on Artur the High prince of the Sea's and Thomas the marine biologist. As well as being caught between two gorgeous men, she is also in the middle of two warring factions of mer-folk, those who are happy swimming in the sea, keeping under the general populations radar and those who wish to "come out" and live among the general population.
This is the second book in the series and it basically pulled me even further into Fred's life, her trials and tribulations. Poor Fred i think I would have a hard time choosing between Artur and Thomas too. Again the warning a bit steamy in places so be careful what age group reads it.
Available at Amazon.com, and Amazon.co.uk £5.59 and Play.com from £2.70