It's fairly ridiculous to expect a precise "working" rhythm to a song when no work is being done with it. It's like if you're dancing with no partner and someone says, "you're not holding him/her tightly enough."   See, the specific work at hand at a specific moment will basically determine the rhythm and tempo, so if people were working why singing it, naturally they would fall into the "correct" rhythm. In other contexts it is very difficult, if not absurd, to mime that.

Don't get me wrong ; I happen to be VERY interested in the practical aspect of chantey singing, including determining realistic rhythm/tempo. I really am! However, I think there is a logical fallacy in a lot of judgements about stage performances. What's more, I think it sometimes gets to the point where people hold this "not correct for work" card over performances like some sort of power play. One can see how a lot of entertainment-based renditions have been ridiculous in style, so as to inspire grunts and sighs about how the performers are fairly clueless. But that has also become a sort of default charge against people who are NOT clueless and -- *gasp* -- maybe decided for one reason or the other, as the situation called for, to do something different.

Certain rhythms, yes, just DO NOT work for chantey tasks, but there are other styles where the rhythm, though you can't dance a jig to it, nonetheless maintains and organizes the labour quite well. It is like the rhythm of breathing, and I think "Shallow Brown" is one of the chanteys that CAN be interpreted in that way. The actions don't occur at precise time points because the effort lasts over a certain interval of time; even at the lowest tempo, it's not really possible to swing a sledge hammer with precise metronomic regularity!