This one is a ‘young adult’ book, about thirty-five years old.
Set in the Mediterranean (I think), it is about a Loch Ness monster/sea serpent-type creature, discovered by the young hero of the story (who may be a Greek sponge fisher). The beastie leaves behind evil-smelling slime and there is a rather tense scene when said young hero is in total darkness in a cave and realises that the creature is in there with him when some of the slime drops on him from above.
That’s all I remember. Any ideas?