Well, the damage done by rams is given in the ship description section.
Arbalests don't really do damage to ships, they are anti-personnel weapons. Ballistae and Springals should do damage to Structure Points.
I would say that Ballisate should do 1d3 Structure Points damage, Springals should do 1D3 - 1D10 damage.
Fire would do 1 SP damage per turn for each size category. Monsters would do damage depending on their SIZ and STR. Storms would do Seaworthiness damage, as stated in the description.