Real Miami is all about canals. The Intracoastal. The residential canals snaking through neighbourhoods. The drawbridges that hold up commuters whenever a yacht passes. Trailer 2 knowingly reproduces this vocabulary.
The canal frames are covered here. For the broader water systems, see the water systems page.
What the canal frames show
Narrow canals lined by houses. Mini docks behind some houses. Bridge crossings over the canals every so often. Boats docked up against the canal sides. Some shots have boats moving through - small boats, not large yachts.
Visual coding: domestic, lived-in, not the marina-style leisure waterfront. The canals aren't part of the city's edge; they're part of its interior.
What the canals enable
Boat traversal within city limits, plausibly. Real Miami's canal system isn't a viable way to get around like a normal commuter, but it does give small boats a real network to navigate. GTA 6 might use this for boat chases, smuggling missions, or just atmosphere.
The other thing the canals allow for is housing diversity - houses with docks are a different type of property than houses with driveways, and Vice City probably has both.