EDIT: Although epoxy makes it a bit difficult to change the battery, depending on how you go about it. Maybe some conformal coating on the PCB + mineral oil.
There's one other good reason to use conformal coating: it has very little change in size when it dries. There are a few spots on the board that would be affected by that, most importantly the load capacitors for the timekeeping fork. Ceramic capacitors have a piezoelectric effect and change shape ever so slightly (they stretch and squash, alternately contracting around their midsection and then expanding). It's not enough to break the solder or the epoxy but if you pot them it changes their apparent capacitance, which will change the frequency of timekeeping. Probably not enough to matter, but still measurable. Large inductors have similar problems and can eventually break loose from epoxy.
I'd suggest a good candidate would be a silicone gel. It can be applied to the watch in liquid form, allowing all air to vacate, then cure into a soft solid once sealed in the watch. It could also be removed relatively easily, if you decided you don't want it in there any more.
Casio says the F-91W is only splash resistant, so you shouldn't wear it in the shower:
https://www.casio-europe.com/euro/products/watches/collectio...
If you want one you can wear in the shower try the BD640W or similar:
https://www.casio-europe.com/euro/products/watches/vintage/b...
I would imagine the best case for a seal would be to have equal pressure on both sides.
Since internal gas is compressible it would be easier for water to enter with a pressure differential. Then, because the water would have to be next to the seal with pressurized gas behind it to exit again, the water would probably stay.
Having a non-compressible liquid inside would make the seal work less in most cases.
Why are people wearing watches in the shower?
You get to forgo taking on/off the watch and you get to wash the watch at the same time, instead of wearing a watch with residue sweat etc back on after the shower...
From your question I take it you don't wear your watch when sleeping either?
You can always jiggle the band a little and let water / whatever flow underneath.
I hung a Casio W96H by its folded strap on the first ring of the shower curtain. It’s been this way for at least 7 years.
How long are your showers?
If not, even with a thin layer, I doubt the result will give you a readable display, as, judging from the red color of the display in the end result, there’s air between the glass and the LCD display in these watches.
There are actual diving watches that are rated for 1km or more. The first image on the Wikipedia page for "diving watch" shows one rated for 3km. Rolex famously attached one of their prototypes to the exterior to the Trieste in 1960, and it survived the trip to the Challenge Deep.