For longer distance shots try using the Landscape mode and the AF auto focus. Some people setup the Macro AF but I never use that setting as it refocuses too much. I only had a Hero 4 before getting the TG6 so cannot compare to the TG5. I think as you see with the turtle video I the AF worked well and did not lose focus. Once inside a wreck it would track different fish and refocus... I think that is one of the things with the TG6 with a fixed lens like the Go Pros you don't need to worry about the focal distance.
This snippet here is a typical example of the auto-focusing issues I was getting when I tested the TG5 out for video - water wasn't particularly particule heavy, average Komodo day...
I was thinking to try it again after locking manual focusing, not sure how that would work for video though.
I'd looked into to it and found I wasn't the only one with similar issues, which prompted me to think that the TG's weren't that great for video use.
Also see this guy here:
For macro video it did work quite well with manual focusing and a tripod - but I'm more interested in shooting wide angle clips, no zooming etc