Closed Loop Coolant Flush/Replacement

I have a 2005 Supra Launch, it has the Indmar Assault with a closed loop cooling system on it. It is not the full closed system, the exhaust manifolds are still cooled by raw water.
Anyway, my coolant reservoir is a little low (the heat exchanger is still full) and instead of just topping off the reservoir, I would like to just do a full coolant flush and replacement. Has anyone ever performed this?
The owners manual recommends that only a certified dealer perform this service; however, I don't not see how it could be that difficult. I typically perform every service on the boat and engine myself as I do not trust my local dealership (they have cut corners on me in the past and I am convinced at this point that I'm a better mechanic than anyone they have on staff). I refuse to believe that I can't just do this job myself but I also can't find in any forum where anyone else has done this, or any instructions online for performing this task.
Any input will be valued.