In another posting, "How to Rescue Someone Drowning" I described "Reach, Throw, Row and Go," the steps that you should take, in their order, to save a drowning victim. Use the same procedures for a tired swimmer, but let's assume that you're now in the water. Let me make the distinction clear: a drowning person is usually in full panick mode. You want to keep your distance. A tired swimmer just needs help. This problem might arise if you're simply out frolicking in the surf with a friend who wearies.
As you approach a tired swimmer ask what is wrong. Offer to swim with the victim. This is the easiest and simplest method. Sometimes, just being next to tired swimmer and offering reassuring words can encourage him to calm down and make it to shore. If the swimmer describes being taken out to sea by a rip current, you can't swim back the same way. Rip currents are shaped by shore bottom conditions. Swim parallel to shore until the current dissipates, then swim back in together.
If the swimmer is rational, and handling the water conditions but simply needs additional propulsion, put her into a tired swimmer carry. Face the victim while treading water. Tell her to place her hands on your shoulders, keeping her arms extended while leaning back and spreading her legs. As soon as she's leaned on you, start swimming a heads up breast stroke to shore. Be careful not to kick too wide since you are swimming between her legs.
If you go after the swimmer from shore, carry a floatation device with you if possible. It can be sometime as simple as an inflatable toy. If you reach the victim with something that floats, you can offer it to him. Rather than tow him in you can tell him to grab the device, get onto his back and start kicking with either the elementary back stroke or scissors kick. You can then guide the victim by swimming next to him, with your hand on his bicep.
Occasionally a tired swimmer will start to panic and devolve into a potential drowning victim. If the swimmer starts to irrationally clamber on top of you, your first priority is to break free. If he grabs your wrists with his hands, you can apply pressure against his thumbs, by twisting against it, leveraging against the inside of the forearm. If you can't talk the victim calm, you might have to put him into a carry.
Use a cross chest carry as a last resort. If the victim is facing you, surface dive, grab each of her legs and spin her around. Work your way up her back. With your right arm, reach over her right shoulder and snug the victim to your hip. Immediately start doing the side stroke to shore. If the victim tries to roll out, keep kicking, but grab one arm with the other to lock her in. When her struggles cease, resume the sidestroke with the victim on your hip.