A beach shoot doesn't have to be a swimsuit suit. You don't even have to go in the water. Though, it's nice to have clothing that matches the environment - linen pants, a breezy top, etc.
Sunset/evening makes it almost foolproof. Plan to start shooting about an hour before sunset, maybe a little earlier if you don't have fill flash. You can very easily put the sun behind the subject and fill in with fill flash, or don't fill in and get a nice silhouette. Do you have a way to trigger your 580EXII off-camera? A radio trigger or a long cable?
You don't HAVE to have the water in every shot - nor do you have to have the sun in every shot. Use the dunes, the sea grasses, etc to make pleasant compositions.
Go scope out some locations beforehand, take a friend along who doesn't mind experimenting with you - that way you'll be experienced and composed when it's time for your senior shoot.
I made a thread to learn about beach photography here - my test results are further down in the thread, but there's good input from lots of other people.
https://photography-on-the.net …/showthread.php?t=1055481