Radar penetrates pure water perfectly fine. But most water on Earth, particularly that in the ocean, has tons of dissolved material which results in very high radio absorption.
This brings up a question of how to communicate with a submerged submarine. At extremely low frequencies (that's the technical term...) radio waves can reach submarines that are submerged below a few m depths. That's why you have things like https://en.wikipedia.org/wiki/ZEVS_(transmitter) blasting at 82 Hz, detectable everywhere on Earth. The US stopped its equivalent (https://en.wikipedia.org/wiki/Project_Sanguine) in 2004, apparently, in favor of "improved" VLF systems.
This brings up a question of how to communicate with a submerged submarine. At extremely low frequencies (that's the technical term...) radio waves can reach submarines that are submerged below a few m depths. That's why you have things like https://en.wikipedia.org/wiki/ZEVS_(transmitter) blasting at 82 Hz, detectable everywhere on Earth. The US stopped its equivalent (https://en.wikipedia.org/wiki/Project_Sanguine) in 2004, apparently, in favor of "improved" VLF systems.
Here's what one of those VLF systems looks like: https://en.wikipedia.org/wiki/Jim_Creek_Naval_Radio_Station