Sounds a wee bit like 'Horse with no name', by America?
Right year, there's a horse, a mention of the ocean but sadly no girl and no horse comes out of the ocean without girl scenario..........so its probably the wrong answer!
Could it be "White Horses" by Jackie Lee. Sounds about the right era and the horse did go into the sea but whether it came out riderless is another thing. If nothing else, it should bring back a few childhood memories.
Thank you Dr Wu and Trevilino, i can remember both of those songs though, used to love "white horses" and "Folly foot farm". I was only 7 or 8 at the time so maybe ive got song all wrong, thanks for taking to time to reply.