Taking a closer look at the Harpoon Quiver artifact & Harpoon Crossbow in Hidden Depths DLC in Minecraft Dungeons. Those hold the ability to shoot at normal speed underwater, whether other land bows might suffer from slower speed. If we want to take advantage of land bow underwater in Hidden Depths DLC levels, we might need to use the Harpoon Quiver (eg. with Harp) so the projectiles (arrows) will travel at normal speed underwater.
Image credit: Mojang AB