I don't think you have a choice in the matter. These are the only guys that run tours
http://www.blueandgoldfleet.com/
Aaron make sure you get tickets in advance. You can't just walk up there the day of or even the day before and try to get tickets. Especially in the summer.
Also, once you get there you can go on the tour on your own pace so if you want to stay longer it's not an issue.