I don't think they're going to work especially well on something that is already wet. Boot dryers are going to do a much better job, or even the poor man's way of using a hair dryers. Silica gel is better for just keeping things that are already pretty dry that way-since they just removing moisture from the air, but don't do anything to promote changing the liquid water in the fabric into vapor (needs heat energy). If you are able to get your boots mostly dry then stick these in, it may help more.
If you're going to use them to try to dry out boots in a locker, you'll probably need to stuff 2 if not more right into each boot to see any effect. OR get silica gel in bulk and make your own packets with coffee filters or a sock or something. The biggest effect is that while it may still remain damp, stuff in your locker might not get as smelly.
The other problem, is they are not going to be effective for multiple drying sessions, you're going to need to recharge them frequently, and also have a ziplock bag to stick them in to keep their remaining drying power.