All depends if you are profittable by other means.

If you have gil, and have income from other sources, and don't need/want Alchemy mats, then by all means, go with grade6+ water, I know I am.

If you are in need of mats/gil more the fishing SP specifically, there are definitely some great fish to catch for profit in grade 5 waters.

Personally, if I want to make some quick cash fishing, I'll make sure it's a full moon, and hit up Cedarwood for some Red Coral. +3's sell alright, especially when people see the +3 Crab bow on the market @ 30mil...