You can get some hand/feet warmers. They are single use, and vary on lasting a few hours, to 6 or more. You can put them in your pockets, and in your shoes. Really, you can put them anywhere. Just don’t put them directly on your skin.
Keeping warm has a lot to do with your limbs. Your body wants to keep your blood in your core. There, it can aid digestion, and keep your organs warm. Blood is easier to keep warm in your core as well, as there is more insulation in your torso. If your feet, or hands get cold, your body will send warm blood to your limbs. This will make you feel warmer, but with less insulation in the limbs the blood will cool quickly, and once it returns to your core, you will eventually become colder, and colder. So. If you can keep your hands, and feet warm, your blood will stay in your core, keeping your body feeling warmer longer.
Hand warmers can usually be bought in bulk. They’re very useful, and easy to pack. I have a few in my bug out bag, in case of hypothermia.
They can be used for transporting reptiles, or you throw one on an outdoor plant with a trashbag over it during a winter storm. They do not use a power source, so you just have to keep in mind when they are supposed to stop being hot.
Many sporting goods stores also sell battery powered self heating socks. I’m not sure how they do in a very wet environment, but I used to sell a lot to duck hunters.