Spotted this promotional code in the new spring issue of Look Local Hamilton magazine (cover features Handmade Market image-Fortinos and many street boxes and local shops carry the magazine).
Use LOOKHHQ20 when buying tickets online in the promotion code field after you pick your cruise and quantity of tickets.

I tried the code for a Sunday cruise after I picked that option. Screen showed 20% off. Not sure if that same discount applies to all cruises or not.