This is going to sound odd, but without knowing the resistance of the frogs, there's really no way of telling if a failure could be of danger to them. Generally anything below 40v is considered safe to humans, but since frogs most likely have more water in them, they will conduct electricity better, and that number could go down quite a bit.
What I would do is read up on GFCIs and see if they will work on a 12v DC circut, or just experiment (post your finding, I'm interested in it too, just haven't had the time to search or try). That's the only way to be sure your frogs stay safe. If something happens and a short occurs it will cut power to the peltier.
As for attaching your peltier, you will need a thermal paste
http://www.newegg.com/Product/Produc...82E16835100007 used for computers and some way of solidly holding it to the Ti. Gravity won't be enough, and peltier's aren't heavy at all. The thermal paste will ensure good thermal conductivity to the Ti and your heatsink, which you must have as well or you will fry your peltier. You'll probably also find that you'll need a fan directly mounted to the heatsink to be able to move the heat effectively. Even then I'm not sure it will be enough because with the peltier inside a confined space it will be difficult to get cool air to it and thus it will overheat.
Making the pipe longer and putting it between two pices of Al or Cu would be a good solution, like you mentioned. Something like this:
Say and 1-2 thick piece on the bottom and a 4-6 inch long piece on the top, both big enough to cover the whole peltier. This has no moving parts and keeps the peltier above the water line. With the larger piece on the top the heat will disipate fine and with a thiner piece on the bottom more heat will be pulled from the water.
Here's a good deal on some Ti:
http://cgi.ebay.com/6Al-4V-Titanium-...QQcmdZViewItem
This isn't bad either, but it's probably overkill and would be difficult to cut without a good metal bandsaw:
http://cgi.ebay.com/6Al-4V-Titanium-...QQcmdZViewItem
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