>>15745948With the Pokedex entry:
"It prefers beautiful shores. With cells similar to water molecules, it could melt in water."
>cells being in any way similar to a molecule>not utter bullshitBut, going by this, Vaporeon does not become water but instead becomes some sort of organic liquid, which may or may not have similar properties to water.
The colon is very good at absorbing liquids (not just water: if you want to get REALLY drunk REALLY fast, pour some beer down your poop chute), but I'm pretty sure it can't absorb all liquids. If we assume it can't absorb Vaporeon-goo, then she'll just slosh around for a bit. It's likely that all sorts of foreign material gets caught up in her when she, say, melts into a dirty pond, so it's likely she has some way of filtering it out when transitioning back to solid form.
Now, worst case scenario would be if she gets absorbed. Now, assuming her cells are similar to cells as we know them, your immune system will not take kindly to 63 lbs. of a foreign cell going every which way (yes, 63 lbs, which is another hurdle to this highly specific fetish.) Even if Vaporeon's cells are not harmful to your body, that wouldn't matter to your immune system: it doesn't recognize it, it doesn't like it.
Depending on how resilient her cells are, she would either be picked apart on the cellular level and killed by your immune system, or your immune system would be fighting a brick wall and you would be in really bad shape as your entire body fights off the infection.
However, if Vaporeon's cells are identical in properties, down to the molecular level, of water, then your body would handle them as it would water. Eventually you will have urinated/sweated/cried out all of her cells. Due to the nature of her ability, I assume that she can survive for extended periods of time even if her liquid body is divided into parts, and would be able to return to solid form once she's expelled.