It seems that someone on the /r/spacexlounge subreddit had the same question:
We know from the FAA filing for the pad abort that Dragon has a fuel
capacity of 1388kg, and we know that it's max landing mass should be
around 8.9 tonnes. If we round that up to 9 tonnes, and then plug in
the specific impulse for SuperDraco of 235 seconds, we get a Delta-V
of 330m/s2.
This is backed up by data from the pad abort. During the pad abort it
reached a max altitude of 1.5km and traveled 2.2km downrange. Assuming
no gravity or drag losses, that would require 183m/s2 of delta-v.
Gravity losses would amount to an additional 49m/s2 given that the
engines fired for 5 seconds.
If we then account for drag losses, and the fact that the abort burn
ended prematurely with fuel still in the tanks, and the fact that it
had to drag the trunk along with it, unlike during landing, then
300+m/s2 of Delta-V seems quite reasonable.
300 m/s^2 is all the delta-v that the Dragon can use to land. From the same post:
Terminal velocity of the Dragon capsule is somewhere in the region of
100m/s. I guesstimated 115m/s, which for a 3G landing burn would
require 155m/s of Delta-V, coincidentally the max velocity during the
pad abort was also about that much. But let's call it 200m/s2 to be
safe.
In the right circumstances, the Crew Dragon COULD perform an emergency propulsive landing using the SuperDracos. I have no idea whether this has been programmed into an abort sequence, given that the capability of propulsive landings for Dragon V2 was publicly cancelled.
The Draco thrusters would likely not be used, given that they are designed for orbital maneuvers and attitude control, and thus would be far less efficient when operated in an atmosphere (as well as the poisonous fuel thing.)
As for your second question, I can't seem to find anything directly about it but I believe that both types of thrusters have water protection, given the capsule's propensity to land in it (and because they are meant to be re-used.)
