Preparing the Ritual
Like all rituals, seabirth has its requirements. These are relatively simple, and the first is merely a social requirement rather than something demanded by magic itself. Anyone going through seabirth should first undergo a period of counselling with trusted healers or elders, typically of at least six months, to be sure that seabirth will be truly healing for them.
Whilst it is possible to go through a second seabirth to make further changes, it is a hassle and an expense, and the mental damage of being changed into a form that does not quite fit in the end would not be worth it.
Thus: discussions happen first, to ensure the individual seeking their seabirth can truly gain what they want from it.
From there, the process is easier. The individual must find someone capable of casting the ritual for them. This does not necessarily need to be a spellcaster, but it most often is - those with significant power over magic are much better at understanding it, and such is the case for rituals like these. They should also find at least four secondary casters, who similarly do not need to be able to truly cast spells.
Whilst secondary casters are not technically necessary, their inclusion is such a boon to the ritual that it has become understood as a requirement over the years.
Ritual Preparations by Hanhula (via Midjourney)
Once the caster and secondary casters are secured, and gemstones to a worth of 20 gold pieces have been found, the remaining components are somewhat more personal: a set of clothing that represents the former, unfitting self, and a set of items - clothing or not - that represents the future ideal. These items are consumed by the ritual, so it is advised to not choose anything too important. These last components allow the ritual's magic to draw more easily on the participant's inner thoughts, aiding the transition.
Additionally, some cultures require the primary caster to collect a piece of turf from the land above to destroy during the ritual. This is said to aid in protection and luck.
Readying the Site
Blåhaj Idol by Hanhula (via Midjourney)
It is customary for the ritual to be performed under the careful watch of a shark idol of some sort. Whilst it is not entirely certain how a shark has come to symbolise transition and change, it has nonetheless been tradition for centuries, and so a statue, knitted toy, or other representation of the shark is set up at the ritual site as the designator of the site's existence.
At its base, as offerings to a shrine, the ritual's components are placed by the primary caster. As these rituals are performed underwater, they are usually secured in place with some kind of netting or are otherwise weighed down.
No circle is drawn to represent the ritual - indeed, it would be hard to do so in the water - so instead, the secondary casters create the circle. At its centre, the primary caster and the target of the ritual float together, hand in hand.
When all of the casters and especially the target are ready, the ritual may begin.
Performing the Ritual
The primary caster, their hands around the ritual's target's, start with chanting as soon as the ritual commences.
They and their assistants, the secondary casters, must succeed in shaping the magic with careful, coercing words and strong performances, re-enacting the struggles the ritual's subject had likely gone through in learning what their new self should be like, repeated once an hour for the three hours of the ritual's full duration. It is not an easy task, but it is worth it.
The rest of the time, the primary caster spends quietly chanting, and the secondary casters call out affirmations of self to the subject. The subject holds their image of their truth tightly in their mind, fixated.
When it is finally done, all casters slump in exhaustion, and water swirls around the ritual's subject. None see the change save for the subject themselves, and when the waters recede, they are changed. They are themselves, but are now fully realised - shaped to match the image they should have been born with.
Seabirth Ritual by Hanhula (via Midjourney)
Typically, this is cause for a long rest for all involved, and then a party for the subject, casters, and friends at which the subject may reintroduce themselves to the world.
I really wanted to comment but words are hard, so I'll just say: this is so well-written and touches the soul, and reading it made me feel happy.