FWIW, most any "welding" shop will have somebody that can TIG weld aluminum. Maybe not everybody there, but somebody there will be their "nice stuff" guy...
MIG welding with aluminum can be done very well, and very nicely. As a matter of fact, the pontoons were very likely pulsed-MIG welded with a push-pull gun (works just like a spool gun, but is smaller and easier to work with, and the larger spool is in the welder, so you don't have to change small spools all the time). Very few places run large scale production like that with TIG. It's about 1/3 as fast, and pulsed-MIG looks VERY similar to TIG.
As for a shop, try and find a local welding shop that makes aluminum truck bodies or something. They will be well set up for aluminum, and any of them will know that it has to be RIGHT when you tell them it's for a boat pontoon. I worked at a local welding/truck body (flatbed) shop in high school, and they had both a couple spoolguns, and a TIG welder, and guys who were very good at both.
Mike