A light for underwater photos and videos is priceless. It’s by far the most important upgrade you can get to really bump up your results to the next level.
Without using a light, most shots deeper than 4-5 feet are simply blue / green. Once you introduce lights, you start revealing the true colors of marine life and the reefs, producing much better images and videos.
I usually recommend a stronger light than 1000 lumens. 1000 and under, should be enough for close-ups, but if you want to shoot things from a bit further away, 1600-2500 lumens would be better and the more the merrier.
As for another camera, I personally think that the GoPro is more fit for video and less for stills. For capturing great still photos, I would recommend a high-end compact camera such as the G7X Mark II, which is very capable and can reach far better results than your average action cam.
Get the G7X III bundle here