One thing that you can do is revise manually the shore line to see if it is connected, which would take you a lot of time. To narrow down the problem, you can create some additional lines (on a copy of you data) to see where the problem occurs.
The part left in blank is the part that has issues. Creating some more additional lines narrows it further down:
Looking at the river you will notice that there is spot that is left open causing the create polygon operation to fail on that line. See below the location. If you close that part, the polygon will be formed.
The suggestion by Anna sounds good, although you will have to see if the precision and the differences between the datasets can be overcome (define which one is correct), because there huge differences in detail:
... and location
And they both have the same spatial reference...