I would say earlier ranks are harder. Once you get up to the point where you can rank in coral head (mid-upper twenties on the fight outside if I recall, low-mid thirties on the sahuagin in the head.) you can usually ride it out to upper 30s. If you're a water shifter or a mariner ranger, by then you can pick apart sahuagin scouts if you're careful. There's also a couple of niche areas with water elementals throughout this level range to help spell yo.

So I'd say high teens - low twenties.

I'd also argue it's a bit harder to rank an evil, because the only sustainable/decent water ranking that I can think of on goodie mobs is spirits. I know storm giants roam aryth or whatever, but I don't consider that sustainable for a few different reasons.

Furthermore I think the addition of ANY level range for water ranking would be cool, because choice is so limited.

Disclaimer: I haven't played a water major or mariner in a hot minute so this is all recollection based and when I happen to group up with them.