Learn to swim.
Swimming skills are the best defense against drowning. Giving children aged one to four formal swimming lessons can reduce the risk of drowning by as much as 88 percent. If you can't do the doggie paddle (at the very least), don't go near the water.

Check the weather report before heading to the beach.
Avoid the beach if there's lightning in the forecast. But if you're already there, wait at least 30 minutes after the last thunder boom before heading back out to the sand.

Look for a lifeguard before getting into the water.
A lifeguard is a "must" in any beach resort. Talk to him when you first arrive at the beach and ask for advice about water conditions, designated swimming areas, and other resort safety guidelines. If you don't spot any lifeguard, request the resort manager for one.

Know what you're up against.
Ocean swimming is different from swimming in a calm pool or lake so be prepared to deal with strong surf before running in. Also keep in mind that the ocean floor is not flat and beaches can change drastically from year to year. When heading into the water, be aware that the ocean floor can drop off unexpectedly, so don't move out quickly without being prepared to swim in water over your head.

Enter water feet first.
Serious, lifelong injuries (including paraplegia) occur every year due to diving headfirst into unknown water and striking the bottom. It's best, therefore, to check for depth and obstructions before diving, then go in feet first the first time; and use caution by always extending a hand ahead of you.

Swim near a lifeguard.
Most drownings occur at unguarded sites, so make note of where lifeguards are stationed on the beach and stay near them when swimming.

Check conditions before entering the water.
Check to see if any warning flags are up or ask a lifeguard about water conditions, beach conditions, or any potential hazards. If you don't know anything or you're not sure about colored flags and their assigned meanings, ask the lifeguard what the flags signify.

Obey posted signs and flags.
Beach signs and flags are intended to help keep you safe and inform you about resort regulations. Read the signs when you first arrive and please follow their direction.

Swim within designated swimming areas only.
Currents will naturally push you down the shore, so make sure that you stay within designated swimming areas. Be conscious of the limits of the designated swimming areas (like by remembering a stable landmark such as the lifeguard's flag or your brightly-colored beach towel or umbrella on shore) and which way the current is moving. Return to that spot in the water regularly so you don't swim astray.

Have young children and inexperienced swimmers wear life jackets in and around the water.
If you're at the beach with a child or adult who can't swim, make sure everyone has a well-fitting lifejacket handy. If you're going boating, every passenger should wear a properly-sized lifejacket at all times.

Pay close attention to children and elderly persons on the beach.
The very young and the very old are among the most vulnerable on the beach. A few distracted seconds of texting or chatting with a friend - instead of watching them - may put them in danger even before you know it.

Keep a look out for aquatic life.
Water plants and animals may be dangerous. Avoid patches of plants. Leave animals alone. Also, barnacles and the shells of mussels and clams can be very sharp, so watch carefully when walking on rocks and move slowly while walking out into the water.

Keep the beach and water clean.
Nobody likes to see the beach or water littered with trash. Even in places where beach cleaning services pick up trash daily, it may linger on the beach for hours, causing an unsightly mess and threatening the health of birds and animals - not to mention your fellow swimmers. So, please do your part. Pick up after yourself and even others.

Swim with a buddy.
Keep a friend nearby in case either of you ends up needing help. If you don't have any, at least have someone onshore watching you.

Never ever mix alcohol and swimming.
Alcohol doesn't only affect judgment, balance and coordination, thus impairing swimming and diving skills. It can also dehydrate you, increasing the likelihood of heat-related sicknesses. Alcohol can also make you take risks at the beach that could lead to serious injury or death.

Never put your back to the waves.
Waves are much more powerful than you think. Even in shallow water, wave action can cause a loss of footing. Stronger ones can cause injuries ranging from simple sprains, broken collarbones, and dislocated shoulders to more serious injuries including blunt organ trauma and spinal injuries.

Watch out for rip currents.
If caught in a rip current, stay calm and don't fight it by trying to swim directly to shore. Instead, swim parallel to shore until you feel the current relax, then swim to shore. Most rip currents are narrow and a short swim parallel to shore will bring you to safety.

Know how to identify swimmers in need.
If someone is in trouble in the water, get help from a lifeguard. If a lifeguard is not available, have someone call 911. Throw the victim something that floats - a lifejacket, cooler, inflatable ball and yell instructions on how to escape the current.

Hydrate often and properly.
Be sure to bring plenty of water down to the sand with you and keep away from dehydrating liquids like coffee or alcohol. Dehydration from extended exposure to heat and the relaxing effects of waves can easily lead to disorientation and reduced energy.

Protect your skin.
Keep blistering sunburn at bay by slathering on a broad-spectrum sunscreen. Make sure too that you have a source of shade - huts, hats, umbrellas, tents - readily available, especially during the sun's peak hours of 10am to 4pm. Remember - eyes can get sunburned, too, so don't forget some shades. Also be sure to bring footwear with you in case the sand gets unbearably hot.

Recognize the symptoms of heat exhaustion and heat stroke
If you (or someone you're with) display any of the symptoms, as illustrated on the upper left diagram, get out of the sun and heat (umbrellas are your friend), follow the advice, as shown in the lower left picture. If symptoms are on the severe side like swelling, confusion, painful and blistering sunburns, it's best to seek medical attention at once.

Sources:
SYMPTOMS diagram from navyadvancement.tpub.com
WHAT TO DO image from www.rhsb.com