In the past I would have said "don't worry" plenty of boots. But I do not say that now.
I found out there are only about 80 pairs of boots on the Silver Cloud and 200 guests - you do the math! They used to use boots that people left behind but for some reason now they even say "you cannot leave boots behind" and that is probably why their supply is not what it used to be. Must have been an issue to change the policies that is usually the reason.
It also depend on what ship and where you are going. On the UK itinerary we did not need boots. Cannot survive without them in Antarctica or the Arctic.