Hey there,

Trying to find a book that I read in the mid to late eighties whilst at school.
Title and Author escape me but it was roughly about two young boys who were trying to view and record a tidal wave(?) from a cliff top.
After the wave had passed, they spotted a cave that the wave had exposed.
When they checked it out they found what must have been a cryogenic capsule of some type.
Inside when they opened it, was an amphibious manlike creature.
They found out that he was from a race that had existed on earth many thousands of years before man.
I have a vague memory of what appeared to be a green manlike creature on the front cover(?)

Hopefully someone out there has read this and knows what the title is.
Haunts me still even after nearly 30 years!