<p>I have PLA and PETG filament.</p>
<p>I hear that 3D filament absorbs water and causes problems when printing but after printing they can be used with water and they are water proof.</p>
<p>So my question is why is it different after printing/what has changed to make it now waterproof?</p>
<ul>
<li><p>Is PETG waterproof or does it absorb water?</p>
</li>
<li><p>Is there a limit on how much water PETG can absorb or will it keep going until it splits and turns to mush?</p>
</li>
</ul>

Filament that absorbs water prior to printing is subject to boiling temperatures as it passes through the heater block. In extreme cases, steam will be visible and a spitting sound will be heard. The filament will expand as the water exits, causing multiple structural and printing problems.
Once printed, dry filament may absorb water from the atmosphere, but is unlikely to be subject to boiling temperatures.
Waterproofing as a general consideration usually means the ability to keep water out, which is possible if the model is sealed and some printing conditions will adhere each layer well enough to the previous one to provide floating-type waterproofing.
PETG is hydroscopic, which means it will absorb moisture from the air. When printed properly (layer adhesion), the model can be waterproof.
These terms are independent and should not be used interchangeably.