Practically everyone is now familiar with the "dot com" top level domain (TLD). We're also pretty accustomed to .gov and .org and we're seeing an expansion of TLDs, including some increasingly specialized designations, such as .diamonds.
Almost every country has a TLD, such as .ca for Canada and .au for Australia.
We liked this domain for the YYZ website because it represents a significant body of water over which we may one day have the opportunity to navigate, it's a British territory and part of the Commonwealth and, finally, because "io" also stands for input/output which has a technological connotation that fits with a dominant theme of our lives.
There's also the simple fact that a lot of TLDs for YYZ have been taken already by people who are connected with or enthusiastic about Toronto's main airport and the progressive rock group Rush. And, if you don't already know why we named the boat "YYZ", you can find out here: