Do you guys think the water called for in bait solution recipes is an important ingredient or just a filler ?

Be hard to say without seeing the formulation. I've seen some formulation they use water in to keep the base material moist to aid in the aging process. Water could be helpful in a bait solution to dilute it in an effort to help get a more uniform mix with the base material...??? Personally there's other materials I'd prefer to use to help get a uniform mix that offer advantages over water
That's my thoughts anyways, take it for what it's worth

It’s on the Nelson formula floating around
Says fill gallon jug up rest of way with rainwater , thats a lot of water

When I make this solution formula I'll add the same amount of rain water as fish oil. Comes up a little shy of a gallon then but it has worked well for me. Maybe the water helps keep the meat moist after mixing?

Im still green at this, but my thought is I dont see the point in adding anything that doesnt have attraction on its own for my own stuff. In my mind, if it doesnt add attraction its just diluting the attractive scents to a degree. Maybe Ive been reading too many posts from Yes Sir!

The water in my bait solution is to dissolve the preservative powder. The water will allow for a quicker penetration of the meat with the preservative.

I agree Sportsman94. Primarily active ingredients. Those in the know may add an ingredient for better ingredient distribution, penetration and/or a bonding ingredient. There are much better materials to be used that will serve more than one purpose. Usually when water is added it is for a volume increase or to act as a wetting/dissolving agent for the product. Most bait solutions usually require the bait meat base to be previously preserved prior to adding a bait solution.

Mine is different. You don't have to preserve the meat before adding the solution. You can pour it into the meat when it has achieved the level of taint that's desired. It will preserve it at almost that point. It takes a few days, but it gets a nice smell once preserved.

Yes I agree in most cases this is Way too much water!! A lot depends on the meat base you are using?
I tried the Nelson bait solution, just too see how it works? Yes it did work and smelled good. But over all I did not like the bait consistency. Too thin, to much water.
A little bit of Hot water does work well for mixing preservatives.
Over all I prefer to preserve the meat base first then add the bait solution.
Paul D. & Bob J. make some great bait solutions. Follow their instructions and you will have some good bait!!