With the world being covered in 70 percent water, there are so many possibilities to use the oceans currents and tides for some source of energy. In my high school biology ecology class, I did a project about tidal energy and I learned about these things that are underwater windmills. How they work is they are put underwater and they rotate from the tides and currents moving. This is a really good way to create energy because the tides and currents are predictable so people will know where to place said underwater windmills. With tides and currents so easy to predict you can make sure each underwater windmill is perfectly placed for maximum energy output.
There are some downsides to these windmills though which is why they are not widely accepted around the world. One reason is that they are so expensive to build and place which is a major factor why they are not everywhere. Another reason is that the windmills will be constantly bombarded with salt water which can cause damage to the windmill which would cost money in upkeep. The last thing that is a downside is it could kill sea wildlife while they are spinning because windmills on land can kill geese so I would expect the same for windmills underwater.
Like I said before it is a great idea they just need to find a way to make it safer for animals and less expensive before it is widely used around the world.