Just so you know, horses can be guided through water with a lead. Just attach a lead to the horse and get in the water. The horse will follow to the next shore.
Note: If you dive deep (3-4 blocks of water) you seem to lose the lead. Just make sure you keep afloat.
If you don't have a lead and can't craft one. Leave the horse in the island, press F3 and take note of the X and Z coordinates). Go back home and craft one, so you can return to fetch your horse.
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Also, Cannibalism: just say no
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Good advice
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