Hi! Boy, I wish I had seen this before you left! I will post my response in case it helps anyone else with the same question.
I had the same dream, too and I got the chance to do this on my honeymoon in Ocho Rios, Jamaica. We went to Chukka Cove Polo Club for a sedate horseback ride ("walk") down to the ocean's edge. We got the chance to walk the horses into the surf which, if you were an inexperienced rider, was a challenge with some of the horses who were not interested in joining you in the water.
I had struck up a conversation with our guide, Michael, and he called me back over to the horses after they had been unsaddled and everyone else had gone to the cabana for a rum drink. He and I then raced the horses through the surf which was mid-belly high on the horses! It was a huge challenge to remain on the horse with the surf pressing against your legs as you move and I often slid right off the back of the horse but I had a ball! My husband said you could hear me laughing on the other side of the island!
I managed to get a couple of canters in around a ring back at the barn and a couple of jumps in under the instructional guide of Michael before we headed back to the ship.
All in all, a great experience. Not sure if that would typically happen with all guides but Michael sure made my dreams come true!