Beast Quest: Zepha the Monster Squid by Adam Blade

Kicking away from the rock, Tom swam towards the golden gleam. But as he reached for the helmet, he spotted movement just beyond. A dark shadow was rising up from behind the rock. It was so huge that it took Tom a moment to realise it was a head with staring, bulging eyes and a mouth like a gaping beak. Seconds later the body followed, with a mass of giant tentacles. Tom could see through the creature's skin, to where three red hearts were pumping.