This trophy requires that you swim a total of 3km with your horse, which is quite a long distance but is a manageable achievement if done in increments. Before you can actually swim with a horse you’ll need to pay for some horse training at a Stable. Stables are found in most large settlements, and you can also build a stable in Ravensthorpe. Once you’ve paid for an “upgrade” for your horse you’ll be able to travel through water on its back!