The thing is NCR needs more resources such as water, the ocean isn't the best option to get that water. So until they figure out some solution I think they will have to keep moving inland where it may be less dry.
When I was in Seattle, it did nothing
but rain, so correct me if I'm wrong, but Washington State isn't exactly what I would call 'dry'. To be honest, I felt that the whole 'water shortage' thing is simply a matter of making mountains out of molehills. The territory that makes up the real-life US is rather blessed, geographically speaking, and I just don't see water as a big issue for the NCR, especially compared to say Saudi Arabia or the Sahel countries in Central Africa. They have plenty of empty, fertile, and undeveloped territory to settle all around them particularly to their colder and less 'dry' north, and they've already expanded into southern Oregon (and as far as we know without facing the resistance they have in the Mojave) Although water is no doubt a concern for them, it hardly seems an existential issue that would convince them go venturing out into indian country looking for water.
Northward and Westward exploration and expansion seem so much more likely (and easy) for the NCR considering the difficulty of controlling these remote, backwards, interior regions as we saw in New Vegas. Win or lose, I really don't see them undertaking a strategic push Eastward, when instead they could take the best harbors on the West Coast and in the Pacific (Hawaii) and establish themselves as the preeminent naval power of the known world for much less cost in blood and treasure. Of course, this is only speculation on my part. The way the Fallout games present their lore purposefully leaves us in the dark about everywhere except the place it's currently set in, so I really don't know what the north is like, and if there are any entrenched power structures there, like in the Mojave.