IIRC, the reason for the locks is not tides, but that one ocean is somewhat higher than the other.
Edit: Correction, it had to do with how deep to cut into the rock across the isthmus. I'm guessing that cutting across Nicaragua at sea level will be quite the undertaking.
Panama Canal - 51 miles from deep water to deep water. Average toll $54,000, and ~15,000 vessels per year pass through, that's roughly $810 million per year.
Nicaragua - 130 miles, 11 years to build at a cost $40 Billion+ dollars. Presuming that it takes simply takes all the business from the Panama Canal and charges the same price, (it won't, but it will allow larger ships, so to make the math simple) that is a 50 year ROI. Yes, the Chinese are known to take the long view.