Try to swim at a beach where there is a lifeguard on duty and don't swim alone.
If you decide to take a dip, know your limits and stay at a depth that you feel is safe for you. Currents can be deceptively strong so keep an eye on the beach to make sure you don't drift out too far without realising.
Tombstoning or jumping into the sea from piers or high rocks is VERY dangerous. There's no way of telling how deep the water below is or what rocks lie beneath so don't take the risk.
Lilos and inflatables can be great fun but they are easily pulled out to sea by even the lightest of currents so be safe and tether your inflatable to something solid on the beach so you can be pulled in again if you drift too far.
If you're taking a trip to the beach, plan your day. Find out what time the tide comes in so you don't find yourself cut off by high water. Don't ruin a fun day out by finding yourself stranded!
If you see someone in trouble in the sea, don't try to rescue them. Look for a lifeguard or dial 999 and ask for the coastguard.
Swimming inland in deep water lakes has its dangers too. Taking a dip to cool off on a warm summer's day may be tempting but the water may be deceptively cold. Plunging into cold water can shock your muscles into cramp, making it difficult for you to swim to safety so be sensible and test the water first.