For offense, how about Warp Wood? 40 feet for a level 3 Druid is pretty much knife-fight range as far as naval combat is concerned, but it bestows the Broken condition on ships. Looking elsewhere in the SRD,
When a ship gains the broken condition, it takes a –2 penalty to AC, on sailing checks, saving throws, and on combat maneuver checks. If a ship or its means of propulsion becomes broken, the ship's maximum speed is halved and the ship can no longer gain the upper hand until repaired. If the ship is in motion and traveling faster than its new maximum speed, it automatically decelerates to its new maximum speed.
In addition, if a ship is warped, it gains the sinking condition when reduced to below half its hit points.
Heat metal would be another good one, if it's cast on a crucial armor plate. You get a curtain of steam which could block the crew's line of sight. The metal plate could anneal, reducing its AC, or it could tear free of the hull underneath.
Ooh, if Heat Metal is cast on the tackle in the rigging (are these sailships or steam powered?) maybe you could set fires!
For defense, use immovable rods as mines. Position them far enough below the waterline that they will scrape against the ironclad's bottom, and cap them with adamantine spikes. If that's not dramatic enough, cap them with Fireball Traps instead.