The commissioners shall make or cause to be made an assessment roll in which shall appear the names of the owners, so far as known, a description of each lot, block, tract or parcel of land or other property, and the amounts assessed thereon as special benefits thereto, specifying separately the benefits from the opening of the waterway, for construction, and for fill if any, and certify such assessment roll to the court before which said proceeding is pending, within sixty days after the date of the order referring said proceeding to them, or within such extension of said period as shall be allowed by the court. In determining the benefit to be assessed upon any lot or parcel of land for the opening of the waterway, the commissioners shall ascertain from the finding of the court or jury whether or not it is remaining land after the severance of land taken from an original lot or parcel for right-of-way of such proposed waterway, and the damage awarded to such remaining land, if any, allowed by reason of the severance; and for such opening shall assess as benefits to such remaining land only the excess of the benefit accruing thereto over the damage awarded by the finding.