Before revision: Ship passage can be limited by the water levels that are too low, when the fairway might become too shallow for typical ("target ") ships, or too high, when it might become impossible for the target ships to pass under the bridges.
After revision: Ship passage can be limited by the water levels that are too low, when the fairway might become too shallow for large", "design ("target ") ships, or too high, when it might become impossible for the target ships to pass under the bridges.