Yes you need to do it pretty much from memory -- you can look down very very quickly if needed, but with practice that usually won't be necessary.
I have one bottle cage on the diagonal tube running from the stem to the crank (the down tube?), so the bottle is slightly in front of (and below) me. That is the bottle I'm actively drinking from. The other cage is on the seat tube (running down from the saddle to the crank). That is the backup bottle. Most of the time I stop in order to switch the back up bottle to the front cage and move the empty to the backup-cage. I can do it while riding but only do it in very safe conditions (no cars, no other riders nearby, flat road).
It's easier to drink while you're coasting or pedaling lightly on flats or small downhills -- times when you're comfortable taking one hand off the handlebar. Be alert to obstacles in front of you. When Mark Warner was governor of Virginia, he was participating in a big ride (I think Bike Virginia) and crashed because he tried to cross a railroad track while he was drinking from a water bottle. (People sent him a bunch of Camelbaks after that.
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- Gray 2010 carbon WSD road bike, Rivet Independence saddle
- Red hardtail 26" aluminum mountain bike, Bontrager Evoke WSD saddle
- Royal blue 2018 aluminum gravel bike, Rivet Pearl saddle
Gone but not forgotten:
- Silver 2003 aluminum road bike
- Two awesome worn out Juliana saddles