My advice is not going to be that different than what a lot of other people have said. Swim Speeds to negate the speed downgrade, and then whatever would be good for your weapon. Is many cases, that will result in going with some Ink Saver Main. But I will break it down by what I usually do with the weapons you are using.
Aerospray - Run Speed Up, Ink Saver Sub
Splattershot - Special Charge Up
Sploosh-o-Matic - Run Speed Up, Special Charge Up
N-Zap 85 - See Sploosh-o-Matic
Rollers in general - Ink Saver Main, Ink Resistance
Specifically:
Splat Roller - Special Charge Up, Special Saver
Krak-On Splat Roller - Ink Saver Sub
Carbon Roller - Ink Saver Sub, Quick Respawn, Special Saver
Flingza Roller - Ink Saver Sub, Special Charge Up
Dynamo Roller - You might be better off just curling up in the corner and crying. Otherwise try Thermal Ink.
Splat Brella - Ink Saver Main, Sub Power Up (if you like the Sprinkler Sub like I do.)
Tenta Brella - Ink Saver Main, a lot of it. Also you should bring with you a lot of hope.
Another weapon class I would recommend trying are the brushes. Neither one of them needs a lot of Ink Resistance due to the recent changes made to them in version 1.4 so I would focus on Ink Saver Main and Special Charge Up.
After all of that is said and done, another thing that would free up a main ability slot is changing your head piece. I know you said that you want to keep the head piece to complete the ninja aesthetic, but hear me out. First of all, the ninja hood isn't the only piece that obscures your face. The bandannas and the face masks do the allow a similar look while allowing other abilities to be thrown into the mix. But what I am getting at is that you don't need to go that far at all. Historically, Ninjas have sported all kinds of looks. Identity obscurement (don't know if this is a word or not) is a classic look. But not all Ninjas necessarily did that. Some Ninjas would take the road of hiding in plain sight by wearing clothing that simply fit in with the culture of the location they were working in. This fit the styles of Ninjas like infiltrators or the infamous Kunoichi quite well. The idea being not that people didn't know who the Ninja was, but that no one knew they were a Ninja in the first place. Above all, remember that traditional Ninjutsu is more about stealth than anything else. So become one with the ink and use it to conceal your true intentions!
I hate sauerkraut is what I'm really trying to say here.