The following which hints at a likely explanation for the video’s apparently bizarre claim that swimming in Lake Ontario may cause you to “have an extra arm or lose an arm”.

Lake Ontario may be the most polluted out of the five Great Lakes. All of the other lakes flow into it, giving it their pollution. It also receives runoff from farms and businesses around it...

It seems it has a reputation for being polluted and dangerous for swimming, although various other sources dismiss that claim, citing water tests that show pollution levels are acceptable. The Canadian woman’s choice of words is a joke based on the urban myth that pollutants cause mutations such as extra/missing limbs.

It is still a very odd thing to say and odd way of saying it - even the urban myth says children will be born with mutations rather than acquire them after a swim. But that nevertheless seems to be the explanation for this usage.

The following hints at a likely explanation for the video’s apparently bizarre claim that swimming in Lake Ontario may cause you to “have an extra arm or lose an arm”.

Lake Ontario may be the most polluted out of the five Great Lakes. All of the other lakes flow into it, giving it their pollution. It also receives runoff from farms and businesses around it...

It seems it has a reputation for being polluted and dangerous for swimming, although various other sources dismiss that claim, citing water tests that show pollution levels are acceptable. The Canadian woman’s choice of words is a joke based on the urban myth that pollutants cause mutations such as extra/missing limbs.

It is still a very odd thing to say and odd way of saying it - even the urban myth says children will be born with mutations rather than acquire them after a swim. But that nevertheless seems to be the explanation for this usage.