No.
I do agree that a toddler who is keeping up a constant babble of noise in the bath can be left out of sight for a very short period. I would sort the washing in the spare room while my daughter was in the bath, because I knew I was a maximum of 4m away and she was constantly verbalising. I would not have gone further away, or downstairs.
But I don't think this is at all comparable to the sea.
If you look at how children/teenagers die in the water, they die because they didn't realise it was dangerous.
You hold a child's hand in the sea partly to keep them safe in the moment. But you also do it so that that child knows the sea is not safe.
I am a very strong swimmer; I learned to swim early. At around age 9, my older brother and I swum out against a rip tide. No one had made us feel we shouldn't. We'd been supervised in water as long as we couldn't swim. Once we could swim, we assumed we were fine.
We very nearly both drowned. I still remember how frightening it was and how hard we had to fight to get out of that rip tide. No one saw us until it was too late; by the time any adults noticed we were already out of the danger zone - but we could so easily not have been. It was a place, and a situation, where other people have drowned.
A toddler will grow out of being in danger when unattended in the bath. A child will not really grow out of being in danger when unattended at sea, because on some level the sea is always dangerous in a way the bath isn't.
You hold onto the toddler so that the toddler gets that message, not just so they're safe right now.