I've been using an older GoPro Hero 3 Black for underwater photos and video. I just did my first dive with a new setup that is an Olympus mirrorless (Micro Four Thirds - aka m43) camera in a housing.
My experience with the GP may not directly apply since the Hero 4 Black should definitely be better. But, my experience has been that the GP does not do that great for still photos (underwater). It does much better for video. However, most of my dives with my GP have been between 60 and 120 feet. For shallower dives, especially in water with good viz (like I would expect in Aruba), it would probably do much better.
All that said, to me it would come down to this: Are you going to use the GoPro for other stuff, or only for diving? If only for diving, I'd suggest to look at some of the dedicated underwater cameras, like a SeaLife or maybe even an Olympus TG-4 (which is good to 50' with no housing and 148' with the Olympus housing, which retails for $299). Having a flash is THE difference (a lot of the time) between "okay" u/w photos and "good" or better. The GoPro has no flash and no way to sync to an external flash. And the GP just doesn't approach the low light performance of a camera with a bigger lens. And using a really bright video light just doesn't come anywhere close to the same.
Any camera with a built-in flash can be setup to trigger an external flash (strobe). With a decent strobe, even a pretty basic camera can get some pretty dang good pictures.
But, if you'd have use for the GP on land, too, then it is definitely a nice action camera!
