The Merman by Dick King-Smith

Zeta is on holiday on the northern coast of Scotland. As she is watching the seals one morning she notices something strange – one of the seals is not a seal but a merman! Marinus the merman and Zeta make friends and he teaches her all sorts of things, including how to overcome her fear of water. Soon the holiday ends and Zeta has to leave. Although she never sees Marinus again, he gives her the secret of summoning the merpeople.