All of the ones I have seen have been for inshore fishing, but I do know that you can get ones that are larger for offshore fishing. Where to get them is what I don't know about. I have heard very good things about them, too. And as Miss Dixie stated, they are very hearty.
You'll have to call around to find who has the bigger ones. I think they'll last about 30 - 45 mins. in the salt water. If you are going to use them, better get a 5 gallon bucket with fresh water and a little aerator from Academy and they should last all day. I have heard guys say they left them in standing water for a couple days and they were still lively. I think they can gulp air like goldfish..... They have worked in the surf for bull reds as live or cut bait.