Yes, a nice sand or pebbles texture under the water, with a gradient layer mask to make it dimmer in the center of the channel. And perhaps add a layer mask to your rocks and paint gray over the ones that should be in the water so they don't look like they're floating on the water.

That's an interesting color choice—it looks a bit luminous right now. I think that would be a cool look for a phosphorescent river in the Underdeep.

Regarding your drawbridge, I would expect it to be constructed perpendicular to the river, so that the bridge is as short as possible. This makes it easier to construct and also quicker to raise and lower.