I'm sure that Thom wasn't considering underwater use, however, he specifically does NOT recommend that lens. Here's a link to his take on a handful of the m43 lenses.
Short Reviews of Micro4/3 Lenses by Thom Hogan
Quick snippet for those who are link averse:
An expensive, dedicated macro lens. I wanted to like this lens, but I kept finding that it didn't live up to my expectations in a couple of areas. Optically, it's excellent. I had no problems with it in this arena, and it produces excellent macro results, too. The problem is focus. It's a slow-to-focus lens in my experience, and a tempremental one when used with continuous autofocus.
I ended up going with the Oly macro adapter and Inon UCL-165 macro lens.