How to clean a carver boat gas tank?

The best thing to do is find a service company in your area that provides fuel polishing (speak to nearby marinas). They perform what could be described as "dialysis" on your fuel and tank. They have a portable machine that will suck the fuel through 1 micron filters and create a jacuzzi effect that will agitate and get out all the sludge and particles (including any water) out of the tank.