Coatings can be useful but to do it with the express purpose of submerging electronics into water is misguided.
Outside of very niche applications, there is no good reason to do it, aside from the novelty when showing it to people who have the mistaken belief that anything electrical will immediately implode when exposed to water.
At the same time you have to factor for the circuit voltage vs conductivity of the water and the sensitivity of the circuit to voltage change.
With typical consumer grade product PCBs there is yet another issue, that quite often the manufacturing process does not completely rinse away the water soluble flux, so once it gets wet again, that creates a conductive and corrosive liquid. For that reason, if you were to DIY coat something, I would thoroughly clean the PCB first, not assuming "new" means "clean".
Particularly an issue could be getting under the BGA chips, especially those with significant thermal/expansion dimensional changes, where if your coating ends up with a gap and water pools inside, you may end up with the flux residue causing problems.
I see it mostly as "look at me tempting fate". Coatings are a 2nd line of defense when you've already tried to mitigate damage from an environment, not supposed to be an enabler to deliberately try to create a problem environment that didn't need to exist.