The majority of waterproof pocket watches are wind-up watches. These watches do not use batteries. Instead, they are mechanical and must be wound before use. This is how they keep time. A wind-up watch has the advantage of not requiring you to replace batteries, which saves you money. Your wind-up watch, however, might occasionally be off by a minute or two. In most cases, this is not a big deal. In general, wind-up watches are very reliable.