Hmmm. Would it be easier for a ghost to get Baptized in a river that thematically fits with their death, or would it twist them less, or something?
Rivers are
dangerous. Most of the time, if a ghost falls in one, they're carried away and lost - either their corpus dissipates, the Whispers get them, or they're swept down into the Labyrinth (depending on the nature of the river, how strong it is and how deep in the Underworld it is when you fall).
Baptism requires the ghost to force themselves to survive and claw their way back upstream. They have to fight physically against the force of the River trying to drag them away (and rivers are hungry - this is symbolically a fight against death itself) and they have to fight mentally against the way that the river seeks to leave them as nothing. The ones that succeed, finding depths of willpower they probably never knew they had, wind up twisted and fused with the nature of the river, their Passions twisted, their corpus warped. The ones that fail (which is the vast majority) are just lost to the river, becoming one with the waters and swept away.
From the point of view of normal ghosts, Greater Dead are as crazy to them as normal ghosts are to humans. Their Passions are twisted, they're not their own. They're scary and powerful and monstrous. Their corpuses are usually notably inhuman, far more than a normal ghost gets. Remember, in our underworld model,
every hun ghost starts at Essence/Enlightenment 1 - whether you were a Solar or a sailor. The po is the seat of power and that's what retains the Essence/Enlightenment value [1]. Hun ghosts are weak - so Greater Dead are scary motherfuckers.
So, no, honestly you'd probably be worse off if you were submerged in a river which matches your own death. After all, that death is already part of you - you're starting pre-twisted. It has all those memories of you dying that it can play off while you're carried off away.
[1] Yes, that means that the hungry ghost of a First Age Solar is basically as scary as the Solar themselves. And that's why you don't fuck with a Solar tomb.