Hi, generally spaeking, engine manufacturers and oil companies do not recommend additives but with the limited use that most sailboat engines get it is a good idea to use a biocide (such as supplied by Stanadyne and Racor) to stop the little critters growing in your tank. I am sure that there are many more suppliers of additives but I have only mentioned two that I have good experience with. 

I have no personal experience with the Valvetect products but am impressed with the info provided on their website http://www.valvtect.com/company.asp . If their diesel additive is already added to the fuel in the correct quantities that you buy from your marina, you should not have to add anything else.
I dont know why one would add marval Mystery oil to their fuel for normal use. This is the type of product that is usually tried to fix an engine problem.
Stanley