lerpy,
You’re right that your training will be more centered around where you get certified; I guess I was lucky enough to get certified on the gulf coast so our first 2 check out dives were in freshwater springs (cold water) and then our 2nd 2 dives were off a boat in the Gulf (warm salt water, current, etc.) so I got exposed to both fairly early and have continued to dive both regularly.
Here though our teaching focuses on diving cold, land locked quarries where the water temp stays about 55-60 degrees year round and we have limited/low visibility.
We do discuss the differences of diving in the ocean, and encourage students to take trips with us to experience it, but until you actually do it the first time it will be a new experience.