There are naval units? Hmm...
How about this: A really big ship-mounted siege gun with very long range, very high cooldown, damages the ship it's mounted on and knocks it back (damaging anything it runs into), and does line damage (both on the way to its target and for a long distance afterwards, outright killing weaker units) and knockback to the survivors?
Also would be limited to one per army, and must be upgraded to an existing ship. Said ship can no longer attack and has very slow move speed.