Any cleaner, even vinegar, is toxic in strong enough concentrations. I place my knotmeter in a cup of white vinegar to kill the creatures attempting to take up residence there.
This is the EPA data sheet for Simple Green: SIMPLE GREEN® | NCP Product Schedule | Emergency Management | US EPA
. Site states there are no known EPA priority pollutents in the product. Simple Green is also EPA-approved for washing up during oil spills.
There are several products labled as safe for boat use including BoatSoap, Biokleen, etc. A trip the West Marine can educate you.