Curiosity inspired by recent vacation travels:

What was the average precision of those 17-18th century cannons
you see in old Spanish forts? i.e.: How many shots would need
to be fired by an average crew to hit an oncoming ship? 
How many hits would typically be needed to sink a ship? 
What methods did they use for aiming?
Did the cannonballs bounce off the water? (That would
certainly make it easier, having to get only one angle
right!)

Has some nut built a firing replica and posted the results
on the Internet yet?

Last year I was at a 19th century coastal fort near San Fransisco. 
According to information at the site, the cannons were indeed fired at
low angles so the cannonballs would "skip" across the water.  This has
two big advantages:

1) You just need to point the cannon in the right direction, you don't
need to "drop" the cannonball right on the ship.

2) You get significantly longer range, because a skipping cannonball
will travel farther than one shot upwards in an arc.

However, you asked about 17th and 18th century techniques, and I do
not know if they used the same methods.

From what I understand, the cannon ball often did less damage than the
flying splinters it created often wounding or killing the crew. I was
fortunate enough to get to see a historic cannon shot demonstration on
Perry's Island (sp). The demonstration was accompanied by some
interesting information, and this is one of the pieces of info I
managed to remember... :)