If you are the only one to ever-ever-ever fuel
your boat and you re-mark the tank as soon as the marker fades the slightest, it should be fine.
Just remember...it all works great until your buddy decides to help you out and goes with the standard convention that red is for gasoline.
It's a standardized convention so that anyone steps on your boat, they instantly know what is in the fuel
jug. Using a marker is like putting up a giant sign at a traffic signal that red is green and green is red...as long as everyone reads the sign it will work.
(probably more critical not to put gas in a yellow diesel
jug as that can trash the engine
. A little diesel in a gas engine
, you just need to flush the diesel out of the fuel system and it should run fine)