The fish in room 11 - Heather Dyer

Toby lives a lonely life in his pyjamas at the seaside hotel where
he was abandoned as a baby - until the day he's sent down to the
beach to look for Cook's lost laundry. Instead, he finds a pale,
thin girl, lying at the water's edge with a long green tail swaying
gently in the shallows.
Eliza Flot is a stranded mermaid and she needs his help. Toby takes
to her like a fish to water, and when he meets her noisy parents
he practically becomes part of the family|
But Toby's new friendship arouses suspicion back at the hotel. If
only Toby and the Flots can come up with a plan to rescue each other|