We replace them every 5 years for preventative maintenance and I want to make sure they don't rust out before then. The stainless bolts are twice the cost and I need about 100 of these things.
I'm a new chemical engineer at a plant that makes nitroglycerin. We transport the NG by gravity through LLDPE tubes inside a trough with warm water to prevent the NG from freezing, (fp=55F, you don't want it freezing or it can explode on thaw out, i.e. the crack when you put ice cubes into a glass of water). The bolts are for the hose clamps that hold the LLDPE in place connecting each building and magazine in our process. We replace the LLDPE and hose clamps every 5 years, I already bought $20,000 of LLDPE tube but the hose clamps are gonna cost me $2-3k more. Trying to reduce costs, but as I expected it looks like SS is the way to go.

Corrosion rates are difficult to quantify accurately. The materials guys almost always hedge by recommending tests.

If your water is fresh enough, probably OK. However, any kind of salt, acidity or alkali in the water likely to trash them.

Coatings like Xylan not good enough for long term direct water exposure.

Personally, I typically go for better materials and lower maintenance costs. Consider extending the replacement period to reduce costs. I find this is actually generally the lowest cost option. Are the stainless bolts really that expensive compared to monitoring/replacements costs for lesser bolts?

Alternately, consider running a test: put a few in where you can monitor and replace if necessary, use balance stainless.

If you can stand the replacement costs, go for it, monitor the bolts, and replace when they start having significant corrosion. The bolt inside the assembly will have less oxygen; consider packing with grease or preservative to minimize internal corrosion.

Finally, you can do small scale tests at a lab for thousands of dollars to see what would happen. My least favorite.

Since the result of failure is great versus the very small amount of money saved, your supervisor should be asking you why you are wasting time on this.