First off good Luck on you'r tig test :) .
Most of all haveing any cert's will help, but i have found that most people you likly will be working for will require you to take a test for them.
Pay depend's on what they see in the test you take at that pointAnd If it's a Shop job or Travle work.
for a first timer, be awhere you'r going to look a bit :"Green" and therefor you cold be looking at a little less pay then some of the old timers.
In the USA . $13 - 17 HR/per Deim (some might pay for lodging as well.. and you might want to look at that before you look at the $HR$ :HINT)
Shop job's most all the time pay a bit less depending on where you live. )
Try taking a look at
http://www.industrialtradesman.com/
for on the road work.
As far as Underwater welding? can't help you there.
Larry