Could the gasoline have flowed out of the tank while the machine was hanging up and down?
That would bring the supply hole, with its perforated screw-cover, underneath.
He waded cautiously into the water and found to his infinite relief that the submerged bank formed a gentle slope.

He could not go far enough to lift his machine, but he could reach to wiggle it off its hook and then guide it, in some measure, enough to ease its fall and keep its damageable parts clear of the water.

At least he believed he could.

In any event, he had no alternative choice and time was flying.

After what he had already done he felt he could do anything.