I answered 1300 lumens with an 8 degree beam but I don't think the answer is really this simple. I have a few dive lights which are in the 1000 to 1200 lumen range at full brightness. They are typically an 8 degree beam. These seem fine to me. A more narrow beam isn't really necessary. I picked the 8 degree beam with the understanding that I could reduce the lumens. All my dive lights tend to have a 100%, 75%, 50% and 25% mode. Even a 100% (1300 lumens), 50% (650 lumens) and 25% mode (325 lumens) would be good.
In clear water I'd want the higher lumens but in brackish water with a lot of backscatter, reducing the lumens is a must. So being able to step down the lumens is important.