Rules and References

Ships in 5E DMG page 119

Castle Walls 247

Siege Equipment 255

Underwater Combat

Hold Breath

A character who has no air to breathe can hold her breath for 2 rounds per point of Constitution. If a character takes an action, the remaining duration that the character can hold her breath is reduced by 1 round. After this period of time, the character must make a DC 10 Constitution check in order to continue holding her breath. The check must be repeated each round, with the DC increasing by +1 for each previous success.

When the character fails one of these Constitution checks, she begins to suffocate. In the first round, she falls unconscious (0 hit points). In the following round, she gains a death failure. In the third round, she suffocates.

Melee Attacks Underwater

When making a melee weapon attack, a creature has disadvantage on the attack roll unless the weapon is a dagger, javelin, short sword, spear, or trident.

Ranged Attacks Underwater

A ranged weapon attack automatically misses a target beyond the weapon’s normal range. Even against a target within normal range, the attack roll has disadvantage unless the weapon is a crossbow, a net, or a weapon that is thrown like a javelin (including a spear, trident, or dart).

Attacks from Land

Characters swimming, floating, or treading water on the surface, or wading in water at least chest deep, have half  cover (+2 bonus to AC and Dexterity saving throws) from opponents on land. A completely submerged creature has total 3/4s cover against opponents on land (+5 bonus to AC and Dexterity saving throws).

Fire

Nonmagical fire (including alchemist's fire) does not burn underwater. Spells or spell-like effects with the fire descriptor create a bubble of steam instead of its usual fiery effect, and creatures and objects that are fully immersed in water have resistance to fire damage.

Spellcasting Underwater

Casting spells with a verbal component while submerged can be difficult for those who cannot breathe underwater. A creature that cannot breathe water must make a constitution check (DC 13 + spell level) to cast a spell underwater. Failure means the spell fizzles. Creatures that can breathe water are unaffected and can cast spells normally. Some spells might function differently underwater, subject to GM discretion.

Casting Spells underwater requires a certain amount of breath. Every Spell you cast takes 1 breath round per spell level to cast to reflect the intricacies of the Spell.

Armor in Water

Light armor can be worn while swimming

Medium Armor: involves making an athletics check at disadvantage with the DC= to the AC bonus of the armor.

Heavy Armor : The Weight and difficulty in wearing heavy armor cause one to sink