Let's distinguish first three things:
- Hygroscopic behavior. The ability of a substance to absorb water. Salt is very Hygroscopic.
- Watertight open-shell. A boat is watertight against water from below. But you can pour water into it. Putting a lid on seals such a body somewhat, but the gap allows water to seep in.
- Watertight closed-shell. A sphere is water tight against water from all sides. You can not pour water into it, no lid needed.
Now, hygroscopic behavior of filament has can create a huge hazzle when printing: Waterlogged filament creates lots of problems, like spazzing, steam, foaming and others. This can result in a watertight designed print being printed in a gappy fashion or result in lack of strength or many other results.
Watertightness of a shell - open or closed - is primarily a result of the design and well fused lines. In my experience, 2 shells can be watertight, 3 shells upwards is almost always watertight. The main problems are with tops, where I experienced that the minimum tops and bottom layers need to be 4, better even 5. But this is dependant on your print quality - and you might need more or less dependant on your settings.
Under stress, like in an RC boat hitting into waves, the inter-layer bonds are the ones that would break first, breaking the shell likewise. To strengthen both the shell and ensure water tightness, polymer lacquers like acrylic paint, guitar lacquer or even epoxy resin (2-component epoxy resin is a very common glue) have been used successfully to create much strengthened and polished surfaces in RC projects. But you don't need to go through these steps unless you really want to.
After printing, a hygroscopic filament still undergoes the same processes and might undergo strength shifts. Sintered-Nylon for example might become heavier and softer if waterlogged. Such material also can't be frozen, the contained water can destroy the print.
For a bath-plug, I just would print the item as a "solid" body from TPU - TPU is after all the opposite of Hygroscopic: it's Hydrophobic. The next best "easy" material is ABS (as you can seal the surface using a little acetone vapor and can withstand 80 °C water for some time without deforming), then PLA (it's barely hygroscopic, but does not like hot water), and likewise PETG will do for some time, possibly better with the heat. Unless you put it in a freezer, it will keep the water in the tub long enough to take a bath.