Once in a blue moon, a giant albino whale is born. These monstrosities take only a year to reach 300 feet in length and hunt down anything they find on the high seas. While a far cry from the threat posed by a kraken or ancient sea dragon, white whales should not be underestimated.

These behemoths can sink large ships and have a gullet big enough to swallow an entire fishing boat. Those who find themselves in the belly of a white whale quickly discover that their troubles have just begun. A white whale's belly is extremely acidic, but the odd sailor has managed to escape from its huge gullet to tell the tale.

Adventurers, hunters, and bored nobles have been known to drop everything when a white whale is spotted. Those who hunt one down are remembered for decades, providing fame and prestige, or enough food, bone, and oil to end a whole village's poverty. Hunting a white whale is foolish and most never return. Others obsess for years after sighting a white whale, until they or the whale meet their end.

Battling Gargantuan Aquatic Monsters

When your players battle an aquatic monster from the deck of a ship, it is normal combat with a few considerations:

If your players are attacking the monster with cannons, they can produce huge amounts of damage per round. Increase the Challenge Rating (CR) of your encounters to compensate.

Decide if the sea monster attacks the ship, creatures on the ship's deck, or a bit of both. If appropriate, adjust the encounter CR to handle the added hit points of the ship that the sea monster needs to damage.

It is always a good tactic to have the creature dive at the end of its turn, only to appear on the other side of the ship during the next round.

Large aquatic mammals with considerable variation between species

As reflected in the statistics above, there is considerable variation between different species of whales and their relative sizes. Larger whales can swallow their prey (or attackers) whole. Digestive juices from the whale cause 1 hit point of damage per turn, and the whale will disgorge prey that attack from the inside, but it may choose to do so in deep water.