It's strange that the phrase "to be in hot water" refers to being in trouble, when soaking in a hot bath is such a luxury!
Indeed, getting some hot water on your skin each day drives the blues and knots away. Soaking in a bath tub is most ideal, but if you don't have one, even a hot water shower or foot bath helps.
Hot water has a great deal of benefits. It improves blood circulation. For people with joint pains and tensed muscles perhaps from a full day of hunching over the keyboard in front of the computer, you will find that being under hot water is soothing and helps to ease pain. While it does not take away the pain completely, it probably has similar effects to things like heat pads and heat rubs that you find in the pharmacies for muscle and joint aches. Water certainly feels nicer on the skin than sticky cream or pads though! Furthermore, it will not trigger allergic reactions. If your feet are aching from a full day of walking (travelers, take heed!), a hot foot bath is the way to go!
After your hot bath or shower, you may find that you are able to stretch further and better, so go ahead and streeeetch those stiff muscle knots away!
Hot water also has a calming and sleep inducing effect. I remember visiting an onsen in Japan once at night. I was very much awake and energetic when I got there, but after a few soaks, I nearly fell asleep in the rest area, and on my way back to my hotel, I found myself nodding off on the train! Not a good idea if you need to be out somewhere, but if you're having trouble sleeping, then a hot bath or shower for about 10 minutes does a lot for calming your body, mind and nerves. For extra soothing powers, add some lavender oil.
If you have a cough or sore throat, the heat from the shower can even help to generate some relief by loosening up your phlegm and mucus. For greater effectiveness, add some eucalyptus oil to freshen up your airways and help you breathe better. Additionally, if you've got the cold, you probably have the body aches associated with it, and hot water can help relieve these aches too. Just dry yourself properly after your bath to avoid making your cold worst.
A few things to note. First of all, make sure the water is at a comfortable temperature. If you make it too hot, you may damage your skin. For reference, the temperature of the hot pools in onsens are typically around 40 degrees celsius or so. If you're not accustomed to hot water, start slow with lukewarm, then slowly ramp it up a little each time. In the onsens, guests are expected to get out of a hot bath and rest for a bit, or go into a cooler pool of water, before they go back into a hot bath. Water is also accessible just nearby so that guests can re-hydrate their bodies. For some people, sitting too long in hot water can cause them to faint, so do be careful! It is important for you to feel comfortable with the temperature, since that's the purpose of the bath/shower!
Hot water can dry out your skin and hair, so refrain from indulging for longer than 10 minutes. If you wish to enjoy a long soak in a bath tub, the water will probably turn cool after a while, so that shouldn't be much of a problem. If not, you might want to consider getting out and washing your face, doing your shaves and what not before getting back into the now-cooler water to soak for longer.
Last but not least, if you have high blood pressure, do note that hot baths can temporarily increase your blood pressure while in it. Pregnant women should also not soak in water that is too hot. Lukewarm would be all right.
Incorporate this quick, inexpensive and fuss-free activity in your daily routine, and it won't be long before you notice some improvements to your body!