I am NOT suggesting that you do that. However, I knew a fellow who inadvertently put gas into one of his tanks. THis was a P/U truck, pulling a 5th wheel trailer, with an auxiliary tank in the bed.
NO place to get rid of it so the advice was run on the contaminated tank on low load, flat areas where the engine was not working very hard.
He had a switch on the dash to control which tank fed the engine. WHen he could see a hill coming he would flip, EARLY, to the straight diesel tank, and climb the hill. When off the hill he flipped back.
It worked and as he burned it down he added diesel untill the tank was all diesel.
He also kept his speed down. Less speed uses less power.
I am not suggesting though that in a boat it is a good idea. His P/U tanks were totally open to ventilation so no fumes collecting. He had the ability to use the contaminated tank under low load conditions and under high load conditions flip to the straight diesel.
You won't have that. You could fill the bilge unknowingly with fumes. You won't be able to flip tanks, at least not easily or readily, depending upon load.
As you use your diesel and then add gasoline you will be adding to an already low tank which means the % of gas to diesel will be very high.
Not good.
If there is any question about range then you should find and use some means of carrying extra fuel. Jerry jugs, drums, bladders, extra tankage somehow. The extra storage has been done many times.