Before jumping into the similarities it is more important at first to recognize the differences.

Similarities
1) streamlined body shape
2) dorsal and pectoral (flippers) fins. Dorsal fins provide stability. Pectoral fins and flippers most likely derived from the arms of land animals.
3) very efficient gas exchange. Fish utilize a countercurrent system of gas excxhange where the blood in the gills flow in the opposite direction to the water. Deoxygenated blood from the body flows opposite of the oxygenated water producing a more efficient from of gas exchange. Whales can take in extemely large breaths in a short amount of time. Compared to humans fin whales take half the time to fill their lungs with air but a whale breathes in 3,000 times more air. Cetaceans hold their breath in order to get as much oxygen as they can. As much as 90% of the oxygen is exchanged during each breath in contrast to 20% in humans.
4) ability to breathe while feeding. Some cartilaginous fish are equipped with spiracles located on the top of its snout. These spiracles take in water for those fish (especially sharks) while it has prey in its mouth. Remember most sharks need to keep their mouths open while moving in order to breathe. Whales have blowholes located near the top of its head. Not only can a whales eat and breathe at the same time but also take quick breaths while jumping out of the water.
5) strong tail muscles for propulsion. Sharks and tunas have much red muscles and especially myomeres. These mucles are very strong and can propel some fish up to 50 miles per hour or more.
6) cryptic coloration and countershading. Skates and Stingrays develop cryptic coloration to camoflage themselves with the sea floor. Killer whales are dark with peculiar white patches to breakup its outline to prey. Both fish and whales utilize countershading as a way to hide itself in open waters. The dark dorsal side matches the ocean floor while the light underside blends in with the light shining from above.
7) migration. Schooling fish and certain cetaceans travel through the same migration routes year after year.

• A small poster that shows how fish use countercurrent to fully reoxygenate doxygenated blood.
• A sample of Shark "eggs" to show
• A picture of a whale giving live birth and pointing out it is born tail first to prevent drowning.
• Show shark teeth and dolphin bones. Ask why there are no shark bones.
• Pictures and diagrams that show the actual shapes of a killer whale, a baleen whale, a shark and a tuna. Note their steamlined bodies. Even add that seagulls and seals have also this kind of body shape. Note the fins and their development from the arms of land animals (or did they lead into arms?).
• For an activity to prove that fin shape aids in propulsion battery operated plastic dolphins can be purchased. Break off the tips of one of the dolphins flippers and have the two race. In theory the dolphin with the wider fins should come out ahead every time. Also these dolphins are countershaded. Although not as popular as they used to be these dolphins can still be purchased through www.toysrus.com.
• Point out countershading in killer whales.
• point out cryptic color for skates or rays.
• A map that shows migration routes of different fish and whales.