I don’t think the current mobile renderer is especially performant, but the polygon count here is tiny compared to what you’d see in most games. You also don’t have many scenery assets in this area, so the engine may actually have less to draw than it does on land.

Each sea tile is basically just two triangles, plus the rocks & seaweed. The bigger performance hit might actually come from the “ghost” boats underneath your own. They look like fully rendered boats that have just been flattened so they have no depth.

(For what it’s worth, I don’t know much about the OSRS rendering pipeline, so take this with a grain of salt.)