According to the D&D online compendium, there are some stat blocks for an Arcane Ballista DDI in Dungeon Magazine 155, page(s) 68.
Also there's an entry for Trebuchet and Crew DDI, Dungeon Magazine 170, page(s) 96.
I'm not sure how useful those are for ship-to-ship combat, though. Look deeper in the D&D message boards - there's a bunch of stuff hiding in there about homebrew ship-to-ship combat - especially between planeships. Specifically the Iomandra and the Dragon Sea variant ship rules.
Shipboard weapons can be attacked independently of the ship they're on, and unlike ships they are not immune to melee and ranged attacks from creatures. A shipboard weapon typically has the following statistics: hp 80, AC 25, Fortitude 25, Reflex 0.
A standard shipboard weapon requires a crew of two to operate: one to load and aim the weapon (standard action) and one to fire and reset it (standard action). The range is given in 100-foot squares (as opposed to 5-foot squares).
Ballista (500 gp): Ranged 5/10; +15 vs. AC (+13 while the ship is bloodied). Hit: 3d10 damage.
Catapult (500 gp): Ranged 10/20; +13 vs. AC (+11 while the ship is bloodied). Hit: 5d10 damage.
Magical variants of standard shipboard weapons are also available.
Magical weapons deal +5 damage per enhancement bonus on a hit, and they often have rider effects such as ongoing damage, damage on a miss, or some other effect. Some magical variants require ammunition, while others don't. Here are some examples of magical shipboard weapons:
+1 flameshot ballista (Level 10; 5,000 gp): Ranged 5/10; +16 vs. AC (+14 while the ship is bloodied). Hit: 3d10 + 5 fire damage, and ongoing 5 fire damage (save ends).
+2 thundershot ballista (Level 15; 25,000 gp): Ranged 5/10; +17 vs. AC (+15 while the ship is bloodied). Hit: 3d10 + 10 thunder damage, and the target ship loses 1 action on its next turn.
+1 shrapnel catapult (Level 10; 5,000 gp): Ranged 10/20; +14 vs. AC (+12 while the ship is bloodied). Hit: 5d10 + 5 damage, and roll 2d6:
On doubles, the target ship loses a number of crew equal to the highest die result.
+2 splintershot catapult (Level 15; 25,000 gp): Ranged 10/20; +15 vs. AC (+13 while the ship is bloodied). Hit: 5d10 + 10 damage. Miss: 15 damage.