Hmm, a very broad subject...
I'll take a crack at a couple things, if none quite fit what you were looking for add a couple more details about your specific situation.
First lets go with the mental side. If your like me then the gears in the old noggin won't quit for nothing. Your thinking about anything and everything all at once constantly. If this is the relaxation you're after you have to find some warm up exercises or routines that help you focus the mind on what you're doing and block out all the other noise. For me, I like to practice whatever I'm doing (diving, climbing, etc.) in slow motion. Keep it shallow for this, the idea is to concentrate on how you are moving. Try to feel each muscle as it is used and concentrate on producing flawless technique (or as close as you can come to it). A couple of minutes of this will usually help to quiet all those background thoughts. It also can be a big help in refining technique. Asuming you know what is good technique to start with...
(speaking of myself here)
On the other hand if the mental stress that keeps you from relaxing is nervousness or fear then repetition may help. If your not comfortable in the water, you'll mostly just have to spend more time there and learn to be comfortable. Stay shallow and only do things you KNOW you can do. Then gradually push a little further. If your worried about safety, spend a day working with your buddy on those techniques. Practice pulling each other up over and over. Start in a shallow pool (2M), then move to a deeper part of the pool. Then practice in open water. This lets you gain confidence in yourself as well as trust in your partner. Note: Make sure you both learn proper rescue technique to start with, you don't want to hurt each other trying to do this.
Now the physical side. If your having trouble getting your muscles to relax, that one is fairly easy. It takes some practice to become second nature, but the results are great. I believe this comes from yoga but it was passed down to me as a general muscle control training technique. To start, lie down on the floor. You can use your bed but don't go to sleep.
Now starting with one foot, deliberately tense it as tightly as you can and hold it for 10 seconds. Then just let go, let the muscle relax completely. Next do the calf, then the upper leg. For each muscle group tense it as hard as you can for 10 seconds then just let it relax as completely as you can. Pay attention to how the muscles feel when tense and when relaxed. After the first leg then do the other. Then do the same with your hands and arms, one at a time. Then move to glutial, abdominal, and finally upper chest and then neck muscles. If you have trouble isolating a muscle group don't wory about it. For example if you can't tense your calf without bringing other leg muscles into play, just go with it. The idea is to learn to control your muscles in as small a group as possible for you. As you go you may find your ability to isolate them increases. You can do the groups one after another and do the whole routine several times. You can also do the same muscle group 3 or 4 times in a row before moving to the next muscle group. With practice you will gain muscle control, and at the same time you should reach a point where you'll be able to just think "relax" and be able do so without the need to first tense up. I have found though that this is something that does not just stay with you once learned. I used to practice this alot when I was climbing, but have not in the last couple of years. Now it takes a bit more mental effor to make myself relax, I can still do it but it is no longer automatic. Guess I need to start practicing again myself. :naughty
I hope some of this helps.
Ed