Not ignore. It's easier to see what we can actually see. To look at mars we just need a powerful telescope or a single satellite and time. To map the ocean we needed an entirely different means of mapping (sonar) since we can't just use light based forms, a method to get that platform within it's limit range of effectiveness, and not kill ourselves with the process. To do it with any kind of speed we would need thousands of ships and tens of thousands of people all working together and almost solely on mapping the ocean floor. Oh, and the ocean floor changes constantly and would cost $3 billion roughly as of 2014. Now, all that being said you'll just need to convince people who as stated Ignore what they cannot see that it's worth the effort and money.
https://www.azula.com/cost-mapping-ocean-floor-2476364201.html
I found her