For the most part, the horses used for these shore excursions are very rideable and docile. I don't think anyone would have trouble riding them. One word of caution however, be sure to take the ship's shore excursion rather than simply getting off the ship and then renting horses.
I had a group in Tahiti a few years back and one of the companions rented a horse on one of the islands, was thrown and hurt her back. Since it was not a shore excursion of the ship's, they required her to get a doctor's approval before they would allow her to board the ship to continue her cruise.
Had she been on a ship's excursion, they would have covered the medical cost and assumed responsibility. In the case of high risk activity (horse back riding, SCUBA diving, etc.), it is my opinion you should always book the ship's shore excursions.
Have a great cruise!