Back in the old days, it was common practice to use a rule that both sides of the Panama Canal were the same sea zone if you owned Panama. But it was always an optional rule that people used (like giving the Axis super powers (Jets and Super Subs)) and never a hard and fast rule.
Your opponent may be working under that premise. However, even if s/he is, (if it’s a she, let her continue, I hate being the only woman in North America who plays this game!) it’s still wrong. Canals are short cuts you can use if you own both sides, but they are still different sea zones on either side.