Yes I can and I used to love swimming underwater....it's a lovely "free" kind of feeling - can't quite explain it.
I would always swim from one end of the pool to the other underwater - of course depending if it was a backyard pool or any Olympic pool!!
I used to be able to, but haven't tried of late. I really don't like putting my head under water, despite being proficient at swimming. Love swimming and I am at the pool every two days doing my laps, but don't really swim underwater.
My brother used to, when he was a teen. It would horrify our mother because she was sure he'd run out of breath! We had a longish pool, & the very 1st pool in our suburb, (built in 1962.) My father had it installed so he could swim a few laps each day.