Old Faithful Geyser in Yellowstone is a famous tourist attraction, blasting hot water and steam more than 100 feet into the air on a sufficiently regular schedule to keep spectators happy. If you ran the hot water through a turbine, you wouldn't get enough energy to supply the Old Faithful Lodge. But, that idea on a larger scale can provide valuable geothermal energy, which is being used in California, Iceland, New Zealand, and elsewhere. Most of our geothermal comes from anomalously "hot" places near volcanoes, and there aren't enough of those to power all of humanity. But, if we were to use "hot, dry rock", pumping water way down, heating it, and bringing it out artificial geysers to drive turbines, an immense amount of energy is available.
Moving water carries power even if it isn't coming out of a geyser. We get reliable power from hydroelectric dams on rivers, and we can extract more energy from waves and currents. There isn't enough of either one to give us all of our energy, but in some places they are greatly valuable, and we can develop new ways to make them more valuable—if you're building a breakwater to protect a city from the rising sea, why not install generators to convert the punishing power of storm waves into valuable electricity for the city?
The heat driving geothermal energy is mostly from radioactive decay in rocks. We have figured out how to generate more radioactive decay, where and when we want, in nuclear fission reactors, which are supplying much of our electricity in many countries. Nuclear energy could generate more electricity, too, although it also generates much debate among those who enjoy its reliable electricity, and those worried about contamination now or far into the future, and about the possible use of nuclear programs to generate material for bombs.
Let's go look at these interesting power providers. We'll save some of the economic and ethical issues for later.