Actually had a similar thing happen to me in my current seed.

No Leviathans in sight around the time I spent from Meadows to the Mountains. Only when sailing around to a Plains destination did I finally manage to find some. First one, on the way there, was alone.. this one let me mine everything without going under.

Later on down the ocean a bit, I've got probably 4 in a row just as you described. Didn't bother mining them out, thought I'd leave them alone as kind of a neat landmark. I'd say all 4 are within rendering distance; 3 at the very least.