I have both an endoscope and an underwater inspection camera. Some thoughts:
1. Get the endoscope. There are so many uses on a boat for them. Wanna see where that wire terminates? Where the cable run starts? What's under the floor? See where the tiny nut fell on the backside of the engine when you blindly took it off the glow plug? Figure out why you can't pull two wires through the arch, but one seems to work? You'll want one that is lit (because most places you'll need to go will be dark), and wireless (so you can use your phone as the display. They're not expensive, and they are SO handy.
2. Underwater camera - meh. I tried an older GoPro. But as one poster stated its wireless capability ends as soon as its submerged, unless you buy something to extend it. Without being able to see what the GoPro is seeing its mostly useless. You'll spend a lot of time hoping to have gotten an image of what you want to see, bringing it out of the water, playing on your phone or on the camera, only to realize you didn't get it and trying over and over. I bought an inexpensive underwater camera from WalMart, around $100. It comes with its own display, which is hard wired to the camera so you don't lose connectivity under water. My use case was to look at the prop for fouling, and inspect zincs during the season to see if I need to replace them - especially in the winter when I can't dive on it. While I've been able to do both, the limited light at the dock under the boat, combined with the murkiness of the shallow bay water, pretty much lets me see only silhouetted outlines. That's enough to confirm the prop is folded and not fouled, and that the prop zincs are still there. I'm using a long boathook and its just OK. Maybe if I had a jointed pole of some sort I could get a better look and have more control over where I can get the camera, but its difficult from my finger pier, and the light angle is SUPER important. If you have boats flanking you and limiting light, you may find its not enough to really get a good look.
Anyway, my two cents.