This page of the Assassin's Creed: Valhalla trophy guide describes how to unlock the Seahorse trophy. This achievement is awarded for swimming a total of 3 kilometers on a horse.
You can't unlock this trophy right at the beginning of the game, because by default, after entering the water, the horse will sink and Eivor will fall.
You have to build a stable and a birdhouse in the settlement of Ravensthorpe - it means spending 400 supplies and 30 raw materials. The construction site is located at the back of the main building of the settlement.
Initiate a conversation with the groom and select the dialog option for mount training.
You have to choose the first training from the list, thanks to which the horse can learn how to swim - it costs 150 silver. You can also optionally pay for training that will allow your mount to swim faster.
From this point on, look for every opportunity to reach new locations on the horse's back. It will probably take you several dozen minutes to get the trophy.