Purchasing an Action camera

Honestly, the new Hero 5 is well worth the $400 price, the voice command and ability to function with no case makes it far and away superior in user-friendliness compared to previous models. I've put mine on a post in the marsh and backed away 50' and was able to yell at it to change modes/shots/etc and it functioned perfectly. So if you're cool with spending $400 on one then that's the one to get.

If you're not interested in those features then you can easily pick up a used 3 or 4 on Ebay in the $150-$200 range and still have a pretty good camera. I say Ebay because I find local listings people always overvalue the hell out of them. Even used Hero 5s can be had ~$300.

I would get a Go-Pro, or one of the similar knock offs. I have a Sony and regret having to go to Sony for accessories. It does well enough filming, just gets expensive buying parts, batteries and stuff.