The Duck Tour is great fun. Like most city tours, you get a great running commentary from a driver who knows his or her stuff, and a potted history of the city. This tour is amphibious (the vehicle is a bus AND a boat, so part of your tour is actually in the river). The vehicles are open-sided, meaning that it's a great chance to get loads of photos.
One of the more irritating parts of the tour is the obligation to "quack quack" at other ducks you pass, and sometimes at pedestrians. I'm a grumpy so and so.
The whale watch is something else. Take care with your choice of operator. There are numerous stalls for different operators and I'm sure they are all reputable, but some offer some serious advantages (they are all about $25/$30 per person).
1. Some operators give you a free ride on another day if you don't see whales.
2. The half-day tours are not necessarily better than the two-hour tours. It might just mean a slower boat and a longer ride out to the Stellwegen Bank. I'd take a faster boat. If the whales are moving around, you have a better chance of a good view.
When we went, we saw quite a few humpback and some finback whales. These are big ones. They may not surface near to the boat, so if you want good pictures, your camera needs to be up to the job. Ours wasn't, so even though we had good firsthand views (which was every bit as good as you'd think), our photos were rubbish.
The tour is led by a guide who stands out front with his/her binoculars (a useful thing to have to hand) and points out whales when he/she spots them. This means you are invariably dashing from one side of the boat to the other. All part of the fun.