are the noticable boundaries between the oceans, so that you can actually tell in a boat, that you've gone from one to another?

from what i have read, the answers is, sometimes yes..
that why there is a indian ocean, and not just a very big south pacific! at some point the water changed and sailor decided it was different enough..

and at places like the straits of magelean, or cape of good hope, (S. africa) the change is oceans is also marked by rough seas, as the different tempuratures, and densities merge.