If you can't swim then join swimming lessons at a local pool or get your mum or dad to teach you in a pool. Swimming is easy to learn when you join. If you are in the pool or sea and you get cramp ect ect do sculling or put your hand up and shout "CRAMP!" im sure someone will help asap.
this is because the sea is full of salt (as most people figure out after swallowing most of it) which acts as a buoyant that's why in the dead sea you float a lot easier as it is full of salt. in the swimming pool it doesn't contain as much salt. hope this helps :)