Here's the water pet info page from magecompendium.
Problem is, it was captured in 2004 and the pet graphic is a luclin elemental, so the stats are probably significantly out of era.
However, it has very firm numbers.
Water pet regen is shown as 6hp/tick up until 39, where it hits 30hp/tick.
http://web.archive.org/web/200408080...r-library.html
This is exactly in line with other pets. Seemingly they are all the same according to the magecompendium all-pets chart:
http://www.magecompendium.com/library/oldpetchart.html
Now, all this being said, it is
very possible these numbers were tnkered with over the years before magecompendium gathered them. But if I was a betting man I'd say P99 is using these numbers right now since they're clear and authoritative.