Hi jkkim! Oh wow, this is really tough question. I think there are definitely strong merits to explore both space and the ocean. However, I think the notion that we know less about the oceans than we do about space is completely out of perspective. Yes we do know less about the oceans than we do about empty space, or even probably stars and galaxies, space is well characterized. But, and this is a big BUT, we do not know more about other planetary bodies than we know about our own oceans. For example, Europa (a moon of Jupiter), likely has the Solar System’s largest ocean underneath its icy surface. We know almost nothing about Europa’s ocean. It could be teeming with life and we wouldn’t know it! So comparing oceans to oceans, our ocean seems pretty well characterized!! So, I may be biased by my choice of discipline, but I think it is much more important to explore other bodies on our solar system than our ocean. In fact, we can learn a lot about our own planet, including our oceans, through comparative planetology – comparing what we learn about other solar system bodies to what we know of Earth will teach us a lot!!
But, of course we should explore the oceans too.
On the asteroid part – the chances are fairly small but they are still significant. It depends on the size of the impact you are worried about. Small rocks collide with Earth fairly frequently, larger ones are much more rare. I am going to assume you are concerned with a major impact – one that could potentially whip out human civilization. Such impacts are very rare. Impacts with 5 km objects are estimated to happen about once every 10 million years. Those can be pretty detrimental to global climate. The last known impact of an object of 10 km or more in diameter was at the Cretaceous–Tertiary extinction event 65 million years ago. This impact is cited as the reason for the mass-extinction of the dinosaurs. The dinosaurs went extinct not because of the impact itself, but instead because of the impact on global weather patterns. So, basically chances are very small. This is especially true for the next 100 years or so for major impacts. Typically we would notice an object that big coming for us with modern technology =) But all the more motivation to explore space, huh?