Many are those who have drowned in the seas of Acqua before their time, and if they were all to rise as undead the sea would be crammed with them. Some greedy ship's captains have gone to sea without enough lifeboats or lifejackets, and those who drowned when the ship hit a rock or otherwise sunk, and who might well have survived had the ship been better prepared, often find that their anger will not let them pass onto the other world, and they are trapped in the sea near the wreck. Only if a priest preforms the proper act to let them pass on, or if they can gather strength by killing a number of humans, drowning them and devouring their souls, will they be able to enter the Otherworld.
They take the shape of a large wave, and the only thing that marks them out as an undead thing rather then a normal wave is that if one looks long enough, one can see a pair of sea-green eyes within it. In a storm they may well be impossible to tell apart from normal waves. As they are made of enchanted water, the only way they can kill is to drown somebody by sucking them down in an undertow, repeatedly breaking upon them to drive them under the water and keep them there. The more of them acting together, the more dangerous they are. Enough of them working together can even flip over a small boat.
Non-magical weapons will just pass straight through them and fire spells and non magical fire will just go out on impact, but ice spells hurt them a lot, and a magical weapon, even one that's sole magic is to glow, will cut right through them,slaying them outright and destroying them utterly, in effect a second,total death.
It is possible to speak to them, if one has the magic that enables one to speak to a water elemential, but they normally have little of value to offer.