Water is not 100% transparent. And if we are talking about a river or the sea, it contains much more substances and particles than pure water. So, light suffers reflection and refraction when entering the water. So, after some meters under water, light cannot continue propagating. We need air in front of our iris to be able to focus. Our eyes werent designed to see underwater or there would be a clear membrane over the iris to be able to percieve depth underwater. Without either of these, you are only able to see a unfocused image. We need goggles or a mask to fix this.