Because the rock is submerged in the water there should be some light waves crushing into the rock. Make a new layer and choose the standard Chalk brush. Lower the opacity to 10% and paint with white some loose lines around the rock where it enters the water.
Open the file "Guy". Use the Pen Tool (P) to create a path around the guy and press Ctrl + Enter to convert that path into a selection. Copy (Ctrl +C) and paste in our document. Don't forget to convert to a smart object! Place the guy on the rock as indicated below.
Create a new layer and place it underneath the guy layer. Paint with a black brush some shadow falling from the guy on the rock.
Create a new Curves Adjustment Layer and clip it to the guy layer. Use the settings shown below to darken - our guy was way too bright for our scene.
Create another Curves Adjustment Layer and clip it to the guy. Use the settings shown below to increase the contrast and further darken our character.
Open the file "Island" and select all. Copy paste the island in our scene and place it as shown below. Use a layer mask to partially hide (by painting with a dark gray in the layer mask) the bottom.
The island is too bright for this scene. Create a Hue/Saturation Adjustment Layer and lower the saturation to -52. Clip the adjustment layer to the island.
Create a Curves Adjustment Layer and also clip it to the island. Slightly lower the red curve down and the blue curve up to increase the bluish hue in the island.
Open the file "Palm". Copy/paste the palm in our scene and place the palm on the island. Create a new layer and place it underneath the palm - paint a very subtle dark shadow (from left to right).
Create a Curves Adjustment Layer and clip it to the palm layer. Use the settings shown below to boost the green colors in the palm.
The colors of the palm are too strong - create a new Hue/Saturation Adjustment Layer and clip it to the palm. Lower the Saturation slider to -46.
The trunk of the palm seems to light - let's darken it. Create a Curves Adjustment Layer and use the settings shown below to dramatically darken. Clip this layer to the palm layer and fill the layer mask with black (Shift + F5). Paint with white in the layer mask of the curves over the trunk to reveal the darkening effect only there.
Select all the palm layers and group them. Click on the group to make it active and duplicate it by pressing Ctrl + J. Right click on the duplicate group and from the drop down menu choose "Rasterize Layer". This will convert the group into a pixel layer. Use free transform to position the rasterized palm layer as shown below.
Create a new layer and paint some waves crashing into the island.
Let's add another suspense element in our scene - a diver that stumbles upon the mighty underwater beast. Open the file "Diver" and use the Pen Tool to make a selection of the diver. Copy/paste in our scene and position him as shown below. Lower the opacity of the layer to 31%.
Create a new layer and set the blending mode to Overlay. Use a round soft brush to paint with white some light coming from the diver's direction (we assume he has a lantern on his head).