OF COURSE , cleaning the fuel does nothing for the fuel tank.
That requires a hand job , a wooden spatula works great , hopefully there will be a proper inspection port ,
if the boat has a real fuel tank, instead of a box of fuel.
The motion of the ocean can easily loosen grunge on tank walls , requiring a different style filter.
The best for large contamination is a sock filter.
These are usually glass , so you can see the level of contamination , with a fine woven sock inside.
Every time it gets about half full, shut down , empty the jar , and hopefully with a gravity setup replace the jar , let it fill , and continue.
Usually only bad bumpy weather will require this procedure.
Check with the commercial fish boat supply store.