The bottom of the ocean is about 7 miles down, as compared to the moon being approximately 240k miles away. The ocean is largely undiscovered in comparison to space and i think there’s a few reasons behind that..

Manned missions to explore the depths of the ocean are impeded by temperature and pressure . The further down you go, the worse it is. This doesn’t preclude unmanned missions to map the ocean. However, satellite and radar wouldn’t work, so we would have to use advanced sonar

Which honestly wouldn’t be any more expensive than our satellite missions to mars and the such. Honestly, i think the big issue is appeal. Space is sexy and the ocean, by comparison is not. I think politicians and interested scientists made a bigger case for space exploration

Despite the fact that there are way more practical benefits to oceanic discovery. Space is the breast cancer of exploration