People are currently exploring and there are a lot of water areas in the Naga section. I highly doubt you would be 'required' to get the Underlight Angler, but like the whole: "Do RtBGs and Arenas to get the Essence of Aman'Thul so you can upgrade your legendaries faster", getting the Underlight Angler might prove useful to you in this raid.
The Underlight Angler requires 800 fishing skill and catching every rare fish in Legion. You don't need to grind it all out at once, since Tomb would probably be coming in like 3-4 months, just if you have an excuse to get started, might as well do it now.
It gives you water walking, and fish form/fast swim mode even in combat and is an all around very useful item inside and outside of other raids, dungeons and outside Broken Isles content (also works in the Old World and the fast swim is faster than the Druid's Seal form).
Like I said, not 'required', but I would recommend getting it as a raider if you want a bit more utility.