We do for tunnels, locks, and if and when we get on to rivers. The logic is that if one of us goes in, the other has enough on their hands to manage the boat for the first few moments, with less worry for the wet one who should at least be floating. Oh yes, and the dog wears one too - ashore around locks and aboard when the boat is moving. We call it her 'boat coat' since 'life jacket' or 'bouyancy aid' seem a concept too far for a dog who hates water, has no intention of falling in, and who would not enjoy being made to practice swimming in it to prove its value to her.
On a similar safety note, having stood beside our boat while it was out of the water for surveying, we realised we wouldn't have a hope of climbing back on from anything like deep water, so we're getting a ladder. There was a previous thread on here about this, and the reality really hit us when we realised that with head and shoulders above the water line, we'd just be able to reach the taff rail supports but not pull our wet clothed bodies up and out. We hope a rope-type ladder, fastened on to a taff rail support and quick-releasable, should do the trick. But rather like the dog and the 'boat coat', we're not keen to have a practice to check it works...