Half a day is enough water. A full day is usually padding.

The walk from Union Station

From the Great Hall to the aquarium doors is about ten to fifteen minutes at a city walk, longer with a stroller or a January wind. You are walking south through the rail lands, downtown Toronto, toward the lake. You can see the skyline while you walk. You cannot see the shark tunnel until you are inside. If your timed entry window is thirty minutes, do not step off a late train and assume the door will wait because you can already see the buildings.

Itinerary A — family at opening

Clock	Move
Opening −25 min	Off the train at Union. Washrooms there. Snacks already bought. The cafe inside is forgettable.
Walk	South, unhurried. If a toddler is already tired, this is your last honest chance to turn around.
Door	QR ready. First slot after opening is when the water still looks like the overnight plant did its job.
First hour	Shark tunnel before the belt fills. Then the jellyfish gallery. Do not reverse that on a weekend.
Second hour	Remaining tanks. Out before nap collapse. Do not add a second downtown attraction on the same morning unless the children are older than the belt is long.

Itinerary B — rainy-day couple

Clock	Move
Late morning, weekday	Rain is why you came indoors. School groups may still be there. Saturday rain is worse, not better — everyone has the same idea.
Walk from Union	Umbrella until the door. Downtown sidewalks puddle. Budget shoes, not the visit length.
Inside	Loop once. Stand in the jellyfish gallery longer than the tunnel. The belt does not reward lingering.
After	Eat elsewhere. The cafe is still forgettable on a rainy day.

Busy days are not for toddlers. I will keep writing that until it is boring. A crush in a dim tunnel is how a half day becomes a story you did not want.