Okay, maybe that wasn't
quite the most informative answer I could have given.
The official rule's Capsize attack appears in the
Dragon Turtle, but I don't care for it much:
SRD said:
Capsize (Ex): A submerged dragon turtle that surfaces under a boat or ship less than 20 feet long capsizes the vessel 95% of the time. It has a 50% chance to capsize a vessel from 20 to 60 feet long and a 20% chance to capsize one over 60 feet long.
I would prefer some variation of the Chont's Ramming attack:
Ramming (Ex): As a standard action, a chont can swim at up to quadruple speed (200 feet) and ram a waterborne target (such as a ship or another creature). To ram, the chont must end its movement in the target's space. This attack deals 2d8+5 points of damage. If the target is a creature, it can attempt either an attack of opportunity or a DC 18 Reflex save for half damage. The save DC is Strength-based.
Upon ramming a ship, the chont can make a Strength check to breach its hull, which causes the ship to sink in 1d10 minutes. The break DC varies with the type of vessel rammed, as follows: rowboat DC 20, keelboat DC 23, sailing ship or longship DC 25, warship DC 27, or galley DC 30. (See Equipment in the SRD for information about ships). Regardless of the check result, every creature aboard must attempt a DC 15 Reflex saving throw. Success means the creature takes 1d6 points of damage from being thrown about by the impact; failure means the creature is hurled overboard.
I happen to already have a Homebrew rule for this very occasion:
Capsizing Ram (Ex): As a full-round action, a submerged ??? can attempt to sink a boat or ship. The ??? can move up to its normal swim speed (#0 feet) and surface under a vessel in an attempt to hoist it upwards until it keels over, or it can swim at up to quadruple speed (##0 feet) and ram the target vessel with the same intent, the impact inflicts #d#+# points of damage to the rammed vessel but also does #d#+# damage to the ??? itself. In either case, the ??? must end its movement in the target's space. The ??? takes no impact damage if it rams a creature, and the target creature can attempt either an attack of opportunity or a DC ## Reflex save for half damage. The save DC is Strength-based.
If it surfaces below a vessel, the ??? must make a Strength check to capsize it with a +## size bonus (total modifier +## for a typical ???). The capsize DC varies with the type of vessel, as follows: rowboat DC 16, keelboat DC 30, sailing ship or longship DC 38, warship DC 42, or galley DC 45. If the Strength check fails with a result 10 points or less lower than the capsize DC (check result 20-29 for a keelboat, for example), then every creature aboard must attempt a DC ## Reflex saving throw (the creature may substitute a Balance check or Climb check for this Reflex save). Failure means the creature takes 1d6 points of damage from being thrown about by the impact and must succeed at a DC 20 Balance check or be knocked prone; if the creature fails by 5 or more they will be hurled overboard if they're on deck. The save DC is Strength-based and includes a –4 penalty.
If a ??? rams a vessel, it makes a Strength check to breach its hull, which causes the ship to sink in 1d10 minutes. The break DC varies with the type of vessel rammed, as follows: rowboat DC 20, keelboat DC 23, sailing ship or longship DC 25, warship DC 27, or galley DC 30. Regardless of the check result, every creature aboard must attempt a DC ## Reflex saving throw (the creature may substitute a Balance check or Climb check for this Reflex save). Failure means the creature takes 1d6 points of damage from being thrown about by the impact and must succeed at a DC 20 Balance check or be knocked prone; if the creature fails by 5 or more they will be hurled overboard if they're on deck. The save DC is Strength-based and includes a –4 penalty.
If a vessel's captain or master is aware of an imminent capsizing ram attack they can try to evade the impact or counterbalance their vessel to prevent it from tipping over. If the captain succeeds at a DC 20 Profession (sailor) check their ship's capsize DC increases to 30 or by +2, whichever is greater; if the result succeeds against the ram's Reflex save DC (typically a DC ## Profession (sailor) check) the ship also takes half damage from the ram and everyone aboard receives a +2 bonus to their Reflex save and skill checks against being thrown about by the impact.
In campaigns that use the nautical rules from
Stormwrack a vessel's shiphandling modifier applies to Profession (sailor) check against capsizing ram attacks, if the handling modifier is a bonus, it adds to the capsize DC if the shipmaster's skill check succeeds. For example, if a vessel has +2 shiphandling a successful Profession (sailor) check increases its capsize DC to 32 or by +4, but a –2 shiphandling does not lower the capsize DC. (Note a ship's
Stormwrack seaworthiness modifier is assumed to be incorporated into the vessel's capsize and breach DCs.)
EDIT: Upon reflection, I'm tempted to add a "launch" between the rowboat and keelboat on the DC progression, although that's a non-SRD vessel. Maybe Capsizing Hoist DC 24, Ramming Breach DC 23? The previous SA uses the same Ramming DCs as the Chont for the sake of consistency, but I'm tempted to tweak the Rowboat down and put the Launch in a gap between it and the Keelboat (i.e. "The break DC varies with the type of vessel rammed, as follows: rowboat DC 18, launch DC 21, keelboat DC 23,…" instead of "rowboat DC 20, launch or keelboat DC 23,…"