salt water should be just fine for muddying stones. If in doubt, use a pump to desalinise (spelling?) the water
a quiver can contain somewhere between 40 and 50 bolts. More than 50 has never been reported in my memory. I do not know if the material of the bolt has any effect.
The bolts assigned in the military meny are per squad. Setting it at 100 for 10 marksdwarfs means 10 bolts per person, though if one has 20, another will have 0. Setting it to 400 or so will allow about 30 to 40 bolts per dwarf. It will be very uncommonfor any dwarf to have 0 bolts, though one with 15 and a couple of others with 45 may happen. Setting it to 1000 will allow all dwarfs to stuff their quivers as full as hey can, plus reserves about the same amount for reloading. I suddenly realize that this may be a factor in marksdwarfs not reloading at times, as they take valuable time to process the claiming of new bolts before they can collect them which could not mix well with nearby enemies, while if they are already claimed they just walk off and get them. Having 4000 per squad, while good to have at hand, might be overkill and actually cause problems when you have multiple squads. A skilled squad can fire a lot of bolts very fast, so 2000 to 4000 supply per 10 man squad may not be a bad thing, but having them reserved per squad sounds like asking for trouble with the game as it is now. Either by one squad claiming all the bolts as Telgin mentioned, or by having squad members walking a long way, because all the nearby bolts are reserved by a different squad, or some other way I havn't thought of yet, the game is inventive/interesting/annoying like that. Personally I set it at 600 for a 10 men squad.
An extra note should be made that there is also a number of bolts reserved for hunters by default. By default, the first 100 bolts you have are for hunters, wether you have any or not, and unusable for defence. If you're not hunting anymore, possibly because the outside is too dangerous and overrun with continuous goblin sieges, ambushes and the occasional zombie invasion added for good measure, you might want to set it to 0 as it takes precedence over squad settings.