[RPG] Does Blood in the Water work if I score a critical hit on something immune to critical hits

From Combat Statistics, SRD:

Multiplying Damage

Sometimes you multiply damage by some factor, such as on a critical hit. Roll the damage (with all modifiers) multiple times and total the results. Note: When you multiply damage more than once, each multiplier works off the original, unmultiplied damage.

Exception: Extra damage dice over and above a weapon’s normal damage are never multiplied.

Thus, you include all modifiers that are just numbers, but do not include any extra rolled dice, like Sneak Attack. The only di(c)e that gets multiplied is the weapon’s base damage di(c)e.

For example, you Sneak Attack for +3d6 with a Rapier (1d8) and have Strength 14 (+2) and Weapon Specialization (Rapier) (+2), your normal attack is 1d8+4+3d6, and a Critical Hit is (1d8+4)×2+3d6.

No, there is no hard limit.

I can’t really cite any rules on this, other than to say that I am quite confident that no one will ever be able to cite a rule that enforces any such limit.

Of course, you’re unlikely to get too many extra attacks from that set up. Maces do not have wide critical threat ranges, and every crit just gives you one more attack so you only keep attacking as long as you keep critting (which you’re unlikely to do). Blood in the Water helps you hit (and therefore confirm critical hits) but it doesn’t help you actually threaten a critical in the first place.

Now, if those were +1 keen aptitude kukris and you had the Roundabout Kick feat, and you took levels in the Disciple of Dispater class, then you’d be looking at a reasonably high likelihood of literally unending attacks. Because you have both Lightning Maces and Roundabout Kick, and aptitude makes those both apply to your already-wide-threat-range kukris, and Disciple of Dispater is a 3.0 prestige class that (in keeping with the way 3.0 worked) expands your threat range in a way that stacks with Improved Critical or keen, well. You’d score a critical threat on most rolls that hit, confirm that on most rolls, and every time you did you’d get two extra attacks. All you need is to crit a few times in a row (not unlikely for you) to give yourself a fairly enormous pool of attacks, that is more likely to expand than it is to shrink.

And if you can find a DM who will let you run that, more power to you. For what it’s worth, Lightning Maces was banned in Test of Spite, which was a very high power arena game played on Giant in the Playground.