If you have a contract that will have you doing bridge inspections on a long term basis, you might want to consider a lease. I don't know of any work boat rental companies in that area but, here in Louisiana, this is common. It might even be cost effective to come down here and get one if you will use it for long enough.
Here's one example of a company that you could go to:
http://www.bayoufuel.com/boatrental/