A waterfront gives you an obvious place to start, but the useful planning question is how much of the city you actually want to cross.
Choose one part of the city
Pick the area you most want to see and make it the centre of the day. Compare your accommodation, your main stop and your return journey on a current map. Two appealing places may not make a comfortable pair just because they are both in Busan.
Leave room for a meal
Decide whether the meal is the main event or a pause in the day. If it is the main event, check the business's current menu, location and booking arrangements. If it is a pause, leave yourself more than one option in the area.
Make the evening optional
If you want to photograph the city later in the day, plan where you can stand without blocking a walkway and how you will get back. Do not rely on a photograph to tell you what the weather, access or lighting will be like on your date.
Spend less time collecting districts and more time noticing one. The photograph here is city context, not a promise of a particular viewpoint or current conditions.