I don't think that we are severely lacking in underwater geological discoveries because there are methods, like core drilling, that tell us a lot about the the bedrock geology offshore. Considerable effort has been expended in doing underwater geological research in support of petroleum exploration.
Where we are missing out is in archaeological information about late Pleistocene human settlement in the New World. It is generally accepted that a lot of late Pleistocene and early Holocene archaeological evidence of the New World diaspora is under water. We see glimpses of these data occasionally when things like Clovis projectile points are dredged up. Clovis points are generally thought to date between 12,700 and 13,400 years ago.
Increasingly, it is looking like humans migrated to the new World by coastlines. If that bears out, the record of their migration would likely be largely obscured by sea level rises during the Holocene.