There is no zeal like that of a convert.
Liir who join the navy often do so against the express wishes of their families, friends and elders. Sometimes they are disowned and ostracized entirely. More often, a Liir who decides to become a "swimmer in the black sea" goes through a death ritual before he enters the service. He lies very still in the water — his loved ones circle him, sometimes for hours, singing mournful songs and touching him one last time, as is their habit when a Liir dies. Then the funeral procession carries his unresisting body to the Naval base and he is remanded to the custody of the Black Swimmers.
It is universally understood that he is no longer Liir from that point onward. He will not behave as a Liir would — he will learn the ways of the black sea and carry out missions which no living Liir could countenance. These Liir retain their old name, although it is likely to have a bittersweet irony. A new name would come if they were ever able to leave the service — such an event is considered rebirth, and would involve a new name to commemorate the occasion and allow the past to be left behind.
A swimmer is brutal not out of efficiency but out of fear. Liir do not breed like flies, nor do they the ability to marshal their entire population as soldiers. They are vulnerable at all times and the first visitor from the stars enslaved them all. A black swimmer does not destroy an enemy world as his opening gambit. It is done as a last resort. Not much a condolence to the millions coughing up plague blood...but there you have it.
If there was a motto of the Black Swimmers, it might well be You must kill — so they don't have to..
In general, recruitment campaigns really are not necessary. Most Liir go to their drowning day out of concern for their race. If they perceive a threat to their people, they put themselves in harm's way. It is what they are socially trained to do.
If the Black chooses to launch a recruiting drive, he only needs to make it clear that the threat is urgent. This can usually be accomplished by rescuing a few civilian survivors from a world or transport vessel that has been attacked and letting them circulate among the general population.
I have become death, the destroyer of worlds. — The Bhagavad-Gita
Training begins on Drowning Day. This is the first day of basic training for a Black Swimmer, the day that a Liir's lungs are first filled with liquid oxygen medium. The psychological impact of those first few hours, during which a Liir must learn to intake liquid rather than pure air, are extremely powerful and devastating. Often new recruits need to be restrained by other Liir, lest they injure themselves in their struggles. Even those who can retain physical control, however, find the experience very emotionally traumatizing, an overwhelming death experience. Thereafter, it is relatively easy to re-arrange patterns of thought and behavior, especially given that the recruits are always volunteers.
Black Swimmers in basic training quickly learn how to communicate in fleetsong and read steelsong symbols, the principles of their drive systems and weapons, combat tactics and maneuvers, and various mental and emotional disciplines and shielding techniques which allow them to cling to sanity and remain on task under extreme conditions. They also pick up the philosophy and mindset of the Liir warrior through telepathic contact with other Liir warriors. All else is learned by experience, as in any navy.
If a Black Swimmer ever returns, and wishes to become a Liir again, he must go through a re-birthing ceremony and he will slowly to be re-integrated into Liir society by use of a long process of purification. The horrors he has seen and done must be cleansed from his body and soul — he must learn to swim and sing again as a Liir. The elders will help him in this process. They are considered strong enough to bear his burdens and each of them will hear one of his battlesongs as a confession and keep the sin of it for him. The heart of an elder is so massive, the long ages of beauty and joy it has lived are so powerful, that one Liir's grief, guilt and rage cannot overshadow it.
Most of the cleansing rituals have to do with being physically close enough to your victims to absorb their death agonies. For a Liir, being within "shouting" distance of a kill is incredibly visceral and devastating. It is hard to put this in human terms without sounding almost deranged — but taking the life of a thinking, feeling sapient being for a Liir is as intimate and terrible as it would be for a human being to drive a short sword into the belly of a close relative, feel that person's fluids rain onto your skin, and stare intimately into the victim's face as life left the body. They not only have to empathize with the death, but taste every shade of pain and rage and despair and fear as that experience devours someone's consciousness.
And they go through this knowing that they have caused it.
They may even have to face the fact that some part of them enjoyed it.
Unfortunately, not every Black Swimmer is able to reintegrate back into society. The Liir are occasionally involved in genocidal wars against other sentients, and are often forced to kill the entire civilian population of an enemy world. The Black Swimmer who must pilot assault shuttles are often the ones most deeply affected by this action. So, how do they cope? Often the answer is "not well". The truth is that, in some cases, a Black Swimmer crew who have seen too much of the wrong kind of action will never return home. When they are no longer needed by their people, they may well choose to "decommission" their own vessel by piloting it into the corona of the nearest star. They understand that they will never be able to become Liir again. In this way, they are still protecting their home from the horrors of war.
With the era of hesitant alliances and peacemaking in the SotS universe introduced by Born of Blood, Black Swimmers may have another way to retire from active duty away from civilian Liir. "Retirement" of former Swimmers to isolated planets held by former enemies gives a Liir warrior the opportunity to return to the water and breathe air again without returning to his own kind — no untainted Liir would be present, and thus no Liir could be harmed by the freight of these war wounds. In addition, it would be possible for such a veteran to perform services for the civilians of the alien population, and thus expiate in daily service some of the sins committed against similar alien civilians and warriors in the past.
It is the sort of solution that might appeal to a certain sort of Liir. Death is not the preferred solution for everyone, and a third alternative to death or an eternity of madness might be very welcome.
It should also be noted, however, that the Liir distaste for war criminals is not simply an issue of "societal context". This proposed exile would be self-imposed. Black Swimmers are profoundly aware of their own unclean nature, and it causes them great pain. They do not decline to return home just because they will be shunned or not accepted. They decline to return home because they know it would be irresponsible and wrong — they are saturated with toxins of a spiritual nature, and they love their own people too much to harm them in this way.
Some of these swimmers may live long enough to achieve 'elder' status on these adoptive homeworlds. Such a swimmer would be the Elder of Earth or of Avalon or whatever human colony that swimmer chosen to inhabit.
Keeping in good Physical health is more than possible, if proper food could be grown or harvested. Procreation would be neither possible nor desired in that population. A Liir unfit to live among its own people is certainly not fit to parent a Liir child, nor would it want to. Such urges would be far more likely to be expressed by nurturing other Liir veterans or non-telepathic alien children.