Use a huge square parallel light, easier to control the pattern and position.
Matching the sun color to the water color will help.
You aren't limited to using one light and the square parallel doesn't distort the gel pattern.
Caustics only show in shallow water and are most defined during early morning and late
afternoon. If I see an image with a whale and there's a light pattern on the ocean floor I
begin to wonder if it's a special mini-whale; )
You can also get better results by increasing the bump on the ground plane.
This type of lighting should be done as the last step in a scene for obvious reasons.
The best way (in my opinion) to make caustic designs/patterns is in the Terrain Editor. So
hit the [E] next to a terrain then Click New in the Terrain Editor. Select from drop down list.
Hit the Fractal button until you see something that resembles an ocean wave pattern. You
want something with very defined peaks and valleys.
[Ridged Multifractal, Slickrock, Ridges, Pocked and Shattered Hills all give good results]
Click the Grid and make the resolution at least 512.
Hit Ctrl+C to copy the image – yes, it's an image – trust me. Exit the Terrain Editor.
Create a square Parallel light. Click [E]
Select Pict Gel in the light lab and when you enter the Picture loader section click Paste
under the first box on the left, Copy and Paste the same image into the Middle box. Exit.
You should now have a decent caustic pattern on the light.
Crank the brightness up to about 100. This is the only time I would recommend a high
shadow value in sky settings also. It defines the light pattern more clearly.
An advantage to a parallel light is it doesn't spread light out all over the scene as a radial
light does. And they don't interfere with each other if you use more than one. (Unless you
stack them on top of each other
The image below shows a fast render using this method (2 lights used)...)

More light stuff...

Bryce 5 Light Lab Shown above