If you want to be a tanker, you'll need more than 2.5k HP in my opinion (my water seer have that much HP, it's an average amount). As for the pet, it depends on what you want it for. I'd use Shasha if I needed extra help sealing and I'd use Xaolan for her Stone Wall skill.

Most people use a wind pet to compliment the water element. Hayate is good for attacking, Shizune for sealing, and Qyala for sealing. Out of the three I listed I would recommend Hayate because she is much stronger than Shizune. I'm training both Shizune and Hayate on my water seer.

Hope that helps. (: