There are several reasons why it doesn't exist presently, but you could be the first one to overcome those challenges! 1. No one lives in the ocean, so transmission of generated power is a challenge. 2. Generators don't get along well with salt water, especially that which is filled with microscopic sea life. The brininess of the water tends to cause rapid deterioration of moving parts. This is combated on naval vessels by using sacrificial zinc, which sailors can replace, but, since no one is manning an underwater generator 24/7, this is an additional challenge. 3. If you remove the power from the ocean currents, it causes there to be less ocean current, which results in climate change. There is no free energy, only energy that is being used for one purpose instead of another. By using the ocean currents to generate electricity, you deplete the kinetic energy that those currents carry. Maybe a little bit has negligible impact on the environment, but, at some tipping point, it would have serious consequences. Since we are terrestrial beings in a terrestrial society, our understanding of ocean current and their impact on the environment is less developed than our understanding of wind currents. 4. Building a turbine is expensive. The hydroelectric generators are very expensive, and those can be installed where they can be directly monitored, and those almost always deal with fresh water. Building the same thing, but out of high-tech chemically-resistant materials, then installing it underwater by a SCUBA construction crew in high pressure suits off of a floating construction raft in the middle of the ocean, building a floating concrete batch plant in the ocean to supply such a place, and then maintaining that equipment as well as hundreds of miles of transmission lines to a coastal or near-coastal management plant would not only be cost ineffective with today's technology, it'd be ridiculously expensive. Maybe someone like Elon Musk would give it a go, since he's used to taking gigantic financial risks.