Isn't anybody able to help me?
I think your question is very open ended - better to ask a question that is a bit narrower.
Have you gone through all the Flow Simulation tutorials? This is the starting point.
I suspect that your heat transfer path is PCBA, housing, seal?, hull and water.
What is the power on the LED?
Is there a seal?
Is the housing Stainless?
Post the assembly.
At 80W this will be a challenge.
Make the hull larger - say 1 x 1 meter. Then make your computational domain slightly smaller so the hull sticks out of the computational domain on all sides say by 10 mm. If your default fluid is air then use Fluid Subdomain and pick the surface on the water side.
First I wold think that first setting up a simple resistor network will give you more incite into where the heat is going and what components to address. When you are satisfied with the resistor network then move to Flow Simulation. Most of the time it is much easier to see the thermal issues with the resistor network than in CFD.
You can contact me at firstname.lastname@example.org if you prefer.