Most of the batteries are the "maintenance-free" type, meaning they may have caps on the top, but may or may not be able to pull them off without damaging the case of the battery. Adding water to revive a dead battery only works if the battery's electrolyte level is low - assuming the battery was fine to begin with otherwise.
If this is the original battery - guessing that it is a Dephi battery - then it is likely the battery is too old. Typically, these batteries have a 5 year lifespan - if conditions are too cold or too hot, battery life can be affected as well.
As for the CCA rating - generally more CCA (Cold Cranking Amps) will yield better starting performance in colder weather. Batteries that have higher CCA ratings also tend to be bigger and/or heavier as well - so keep that in mind. To get the right battery - make sure you note the BCI group number of the battery. I believe it will be a BCI number 35, which specifies the physical size of the battery. If the battery looks too tall - you'll have to remove the bottom spacer, if it looks too short - then you need to add the spacer back in.
I also agree with your choice - if those were the two to pick from, I'd go with the 500CCA one, the 540CCA spec's doesn't seem right to me, might be a misprint on the website. Need to check on that in person.