Jack Russell Terrier bolting

(USA)

I took your online course when I first brought
home my JRT last February. I think he has done remarkably well for only being
here for less than 5 months. His vocabulary is terrific and because he dotes
on my, he listens very carefully to what I say except . . . I cannot get Tucker
to stop bolting, and subsequently taking off for parts unknown. I have trained
him with the words: come, sit, stay, down, off. He's a bright dog, but its
been very frustrating to have to litterally wrangle him where ever we are when
he gets loose, just to get him either back home or at least safe. We've taken
him on our boat and he is a gem, but once back on his leash on dry land, he
slips his collar and took off, again. I was frantic of course, watching him
tear off for the street. Please let me know how else or what else I can do to
prevent this upsetting behavior. Thanks in advance.