Well how about making siren caste /if there is one ofc, havent really readed everything/ could make their voice be used as magic source? I know charming and such things are succ domain, but making enemies drowzy for example or giving extra good thoughts to nagas that talk with them?
Also i would as player like to see nagas make use of shells and other sea's offerings better than other races (just like dwarves with stone).
Give unhappy thoughts when water is frozen (well i think its ok since its their theme)?
Also if anybody played DotA, there is hero called Gorgon, maybe it would make good archer caste?
Also going this way, would it be possible to make their hair to be snake-like?/that's for Gorgon caste idea/
Also since they have something in common with snakes (well if these are fish-focused naga race, maybe make caste that has snake tail instead of fish one and be more of liar/toxicologist) let them have special kinds of poisons for their trap instead of fire and stone-focused ones?
i hope all i wrote is understandable enough since its kinda late here and i've just got some ideas to share with you