Lower transparency under water
First a little bit of explanation. Before the rise of the digital camera you had to take your film to a photo shop for development and printing. That this photo shop did a lot more to get good results was not important to you. But nowadays things have changed. Pressing the button is still the same but now the picture is instantly developed. So the only thing that is missing is printing (or publishing on the web). If this is all we do we skip the steps that previously were done by the photo shop but are surprised that the result is not optimal. The younger generation grew up with the digital camera so they don't have a reference from the film period. The problem is that the photo shop did additional work to get a good result and it is this work that we have to do ourselves now. Developing the digital image is not necessary unless you take RAW-pictures. But some corrections are still necessary to get a result that corresponds to what you aimed at when you took the picture.
With almost every digital camera comes an"editing" program. Editing programs can also be bought separately. Photoshop CS is such a well-known editing program. Photoshop Elements is a simpler version of this program and that is sometimes included with the camera. Photoshop CS is a far more expensive commercial version. But expensive or cheap all programs have at least the option to correct contrast and color.
The above image is processed in Photoshop CS with the function "Autotone". That is function that with one click corrects contrast and color. The result is far better than the original greenish picture. This is how I remember the situation under water.
Because of the lower transparency a good tip is to get as close to the object as possible. It is for that reason that there is almost no space around the main object in the above picture. Most beginning photographers tend to take pictures from a distance and to zoom in to get the object better framed in the picture. This zooming in results indeed in the object filling the frame, but the amount of water between camera and object is more than is necessary. Try to "zoom in" by moving towards the object. Keep the camera at wide angle (fully zoomed out) and move towards the object till it fills the frame. Then the amount of water between object and camera is as little as possible.