That is typically what I did in previous editions - and I've been DMing since the late 1970s. However, those would typically be one encounter with one group of monsters or bad guys. For example: On a three week journey by sea, a group of sahuagin attack the ship. Works well as one big encounter, but not quite so well as 3-4 waves of attacks.
Maybe I misunderstood the question, perhaps your question is, how to make a one encounter day interesting. Or perhaps I'm still misunderstanding...
There are several ways to go about this as well.
Don't allow extended rest. Perhaps the sea is churning all night, there is a constant storm, or the ship crew is rowdy, and won't let you get in a good night's sleep. Or if they are in a jungle, constant rain through the night, or the howling of wolves, or some major threat that forces them to move around constantly may prevent an extended rest. You could even reward a partial benefit if and where needed, like everyone recovers a healing surge or some such.
There are also those who run games, redefining the meaning of extended rest to be "a week's rest", or "in between adventure's rest." This allows you to have as many time as you want pass by between encounters, for it to make sense to your story. I haven't tried this approach, but it works for some.
Another way is to make the noncombat encounters through the day, costly. There could be grueling tasks, that cost healing surges, force people to expend daily powers, and the like. You could have exhaustion set in. And just when things seem to calm down, you could spring your one combat encounter of the day.
And my preferred method... don't make one encounter days

Even if there is supposed to be combat, you can narrate through the party sailing through Sahuagin infested waters, and skirmishing several groups of them along the way without major losses. It gets the point across without meaningless dice rolling.