2 years old doggy First time go into ocean

It is said dogs are good swimmers. I got a small water tub at summer time for Chance to survive from the extremely hot days, but she refused to go in. It is unbelievable she is re-homed from an island and I take her to the beach, she hesitated for quite a while and watched other dogs having fun in the water. Eventually she went to water but she is so nervous.