I am sure that you will have plenty of time after the excursion to see and do your own thing, When we went, we had the latest time and that cut us close on the way back...we were on the last or one of the last tenders back to the ship (but we felt safe because it was done thru Disney themselves). I think that since you are doing an earlier time that you will be just fine. I think the swim itself is only about 45 minutes (if that) and some people do the swim before the turtle farm and some were vise versa. They put you in a shuttle (van) and take you straight there and then they drop you back off right where you picked the shuttle up from. Im sure that if you wanted to, you could take a taxi back (as I did see some people get in a taxi from there) so its up to you. You will enjoy it, not bad for the cheaper price
I will warn you though, the pics they sale are expensive and they do not have any package deals, its only per photo...and I want to say that they were like $16 a photo (dont quote me on that one because I cant quite remember exactly) but they were expensive (or so I thought considering the price was per photo and no photo package deals). They give you lockers there and there is a place to change clothes if you need to do any of that
Hope this helps...let me look thru my photos now to see if I can find any pics from it for you...