Waves

Like as the waves make towards the pebbled shore,
So do our minutes hasten to their end;
Each changing place with that which goes before,
In sequent toil all forwards do contend.

Does Shakespeare mean exactly what he says here? It's hard to credit it: because minutes, we know, are cumulative in a way that waves on a shore are not (or the waves would eventually close over Ben Nevis). Or to put it another way, does Shakespeare mean, in a sloppy sense, 'the waves appear to keep coming and coming, and that's just what time is like'? Or is he being more precise? As it might be: 'you may think the tide comes in and goes out again, because that's what it has always done; but a tide is coming soon that will never recede ...' Or, more cannily still: 'minutes appear to march inexorably on, but I know a deeper truth: in fact time is a rocking, not a progression: times throws itself in upon us, and then it withdraws again.'