Well, we ought not give up on the ocean yet. There's still a lot to see down there. Who knows, there may yet be valuable resources that we have missed, and they'll be a lot easier to get than resources in space.
Not really. There are plenty of resources (oil, minerals, etc.) but a huge piece of the population would fling stuff at the people for doing that, looking back upon the BP oil spill in the Gulf of Mexico. Not to mention the fact that there are limited resources on Earth, and it'd be more practical to just give the ocean the 1%.
Plus when the shallow resources ran out, it'd be costly to make, for instance, a submarine to mine the minerals on remote control (or manned, although this soon would turn out the same way with manned space missions) that is supported at a high pressure. Keep in mind there would be little to no profit, and not many entrepreneurs work for the well-being of the world.
Space flights would essentially have the same problems with pressure and profits, although there are virtually unlimited resources out there. Also public opinion would support it more, because for one there hasn't been a chatostrophoc eco disaster caused by the production or flight of spacecrafts, and also humans naturally are amazed by scientific advances, and want to see more.
Also, like Tony said, there are planets out there made practically purely out of rich and useful metals, rather than a small deposit.
Environmentalists (emphasis on mental) are the bane of profitable and knowledgeable progress in many cases.