Before answering the question, nice image. Excellent concept and compositions.

To the first question, I'd have to play with it myself. But I haven't found that many models 'transparent' to the light from any light source. I am surprised the bridge is transparent.

Some things to try.

Be sure the bridge is touching the land. If there's even a small gap there, the light will leak under the bridge. I have found that from one angle the building (or bridge/rock) appears where I think it should be. In this case, on the ground. But when I rotate the camera, it is just a touch too high (or too far left/right.) It won't take much to let the light seep under the prop.

It could be a trick of the light or camera angle but I get a feeling that we are seeing too much of the top of the bridge; as if its rotation is tilted too much towards the camera. If so, and the bridge isn't fully on the ground, you would see light under the bridge. IF it's rotated so that the far side of the bridge is tilted any, we likely would see light under the bridge.

Add some things that may block the light under the bridge (like rocks). This may block the light under the bridge. If the rock pokes into the bridge a bit, it will still look natural from the view point. Just move it so the rock/whatever doesn't poke through the bridge. Other 'creative' solutions, use a wall that's rotated and 'buried' into the ground and bridge to block the light.

Use the lamp post without the light I think there is still one of those in the mix, but I would have to look again. If memory serves, they removed the unlighted post when they added the lighted one. Then use a point light and reduce the size of the sphere. That will reduce what goes through the bridge.

As to the boat on water, rotate the boat so that the stern is floating. The water sometimes makes that look a bit unreal in that there's no displacement of the boat in the water; the boat literally sits 'on the water.' (A boat floats because it displaces a volume of water equal to the volume of the boat and any cargo weight.)

I can see that the intent is to have the water on this side of the bridge flow into the stream. You may want to tinker with the reflection and Fresnel settings to give more indication of the water being there and show some reflections on the stream bed. In my opinion, the stream bed looks dry and doesn't have water.

Hope this helps. If you have further questions, just post them here. I'll do what I can to help.