Crater Lake National Park's major feature, of course, is Crater Lake itself. The following information about the lake is from the article about the lake and park on Wikipedia:
Crater Lake is a caldera lake in the U.S. state of Oregon. It is the main feature of Crater Lake National Park and famous for its deep blue color and water clarity. The lake partly fills a nearly 4,000 feet (1,220 m) deep caldera that was formed around 5,677 (± 150) BC by the collapse of the volcano Mount Mazama.
The lake is 5 by 6 miles (8 by 9.6 km) across with an average depth of 1,148 feet (350 m). Its deepest point has been measured at 1,949 feet (594 m) deep, though as with any lake its depth fluctuates with the climate, particularly rainfall. This makes Crater Lake the deepest lake in the United States, the second deepest lake in North America (Great Slave Lake is the deepest) and the seventh deepest lake in the world (Lake Baikal is the deepest). The caldera rim ranges in elevation from 7,000 to 8,000 feet (2,130 to 2,440 m).
Mount Mazama, part of the Cascade Range volcanic arc, was built up mostly of andesite, dacite, and rhyodacite over a period of at least 400,000 years. The caldera was created in a massive volcanic eruption that lead to the subsidence of Mount Mazama around 5700 BC: about 50 cubic kilometers (12 cubic miles) of rhyodacite was erupted in this event. Since that time, all eruptions on Mazama have been confined to the caldera.
Lava eruptions later created a central platform, Wizard Island, Merriam Cone, and other, smaller volcanic features, including a rhyodacite dome that was eventually created atop the central platform. Sediments and landslide debris also covered the caldera floor.
In time, the caldera cooled, allowing rain and snow to accumulate and eventually form a lake. Landslides from the caldera rim thereafter formed debris fans and turbidite sediments on the lake bed. Fumaroles and hot springs remained common and active during this period.
Also after some time, the slopes of the lake's caldera rim more or less stabilized, streams restored a radial drainage pattern on the mountain, and dense forests began to revegetate the barren landscape.
Some hydrothermal activity remains along the lake floor, suggesting that at sometime in the future Mazama may erupt once again.
Due to several unique factors, most prominently that it has no inlets or tributaries, the waters of Crater Lake are some of the purest in terms of the absence of pollutants in North America.
Secchi disk clarity readings have consistently been in the high-20 meter to mid-30 meter (80-115 ft) range, which is very clear for any natural body of water. In 1997, scientists recorded a record clarity of 43.3 meters (142 ft).
Coordinate given are at the rim of the crater of Wizard Island. Boat tours of the lake are available and there is an option to stop and spend some time on the island. The view of the lake from there is gorgeous.