We still have Hoku on a long line, but frankly it's not necessary.
I highly doubt Spot would swim out to sea. But if you are worried, do the long line thing first. Or can you try him first in a lake? Also, what is the sea like where you are? Big waves? Currents?
I did, years and years ago, witness a dog—a doberman—who swam way out far, and just played in the waves for half an hour or so. It was crazy. I was sure the dog was confused or in trouble, but he / she wasn't. They were just having fun.
That's not going to happen the first time a dog goes into the water, though.
Good luck! Hoku had not been in much water—she'd go in, but just wade to her belly—until a few weeks ago, when she jumped off a dock and swam around a bit. She came out happy and surprised at what she had done!