This is more of an expansion on what other answers have already stated, but I figure it'll best serve as its own answer rather than a comment.

The major factors surrounding failure points of waterproofing are, as has been said, the ingress of water around the main case opening and any I/O areas. For the main case opening, this ingress is usually mitigated by an O-ring. Due to the deformation of the O-ring under pressure, hard, 90$^\circ$ bends usually are the primary point of failure. This is the reason why most case seals are given a fairly wide-radius (with respect to the channel itself) rounding on the corners.

To give perspective for better seals, high-vacuum connections usually use copper for gaskets; CF flanges have a "knife-edge" that bites into the soft copper gasket, creating a leak-proof seal at high-vacuum, large-differential pressures. If you intend for deep immersion and have access to the manufacturing needed, this may be the route to go.

As to the issue of I/O interfacing, most deep immersion systems use fully-isolated bulkhead connectors, where the cable itself does not penetrate through a hole in the case. The actual transmission, for example, would be through a female-to-female connector electrically connected and then "potted" using thermoplastic, epoxy, etc. into the wall of the bulkhead.

As to the issue of phones specifically, the problem here is that while any sort of touch-screen layer can be made arbitrarily ingress-resistant, the layer of touch-transmittable material is often quite thin and thus not suitable to much more than casual water immersion (i.e. low water pressure).

And finally, the issue of wood itself as a waterproofed case material; certain, denser hardwoods are notably less porous than others, but this can be increased by applying a layer of sealer such as painted-on epoxy or polyurethane inside the case. Still, FWIW I wouldn't be putting my phone in a wood case and throwing it in the pool.

The major factors surrounding failure points of waterproofing are, as has been said, the ingress of water around the main case opening and any I/O areas. For the main case opening, this ingress is usually mitigated by an O-ring. Due to the deformation of the O-ring under pressure, hard, 90$^\circ$ bends usually are the primary point of failure. This is the reason why most case seals are given a fairly wide-radius (with respect to the channel itself) rounding on the corners.

To give perspective for better seals, high-vacuum connections usually use copper for gaskets; CF flanges have a "knife-edge" that bites into the soft copper gasket, creating a leak-proof seal at high-vacuum, large-differential pressures. If you intend for deep immersion and have access to the manufacturing needed, this may be the route to go.

As to the issue of I/O interfacing, most deep immersion systems use fully-isolated bulkhead connectors, where the cable itself does not penetrate through a hole in the case. The actual transmission, for example, would be through a female-to-female connector electrically connected and then "potted" using thermoplastic, epoxy, etc. into the wall of the bulkhead.

As to the issue of phones specifically, the problem here is that while any sort of touch-screen layer can be made arbitrarily ingress-resistant, the layer of touch-transmittable material is often quite thin and thus not suitable to much more than casual water immersion (i.e. low water pressure).

And finally, the issue of wood itself as a waterproofed case material; certain, denser hardwoods are notably less porous than others, but this can be increased by applying a layer of sealer such as painted-on epoxy or polyurethane inside the case. Still, FWIW I wouldn't be putting my phone in a wood case and throwing it in the pool.