This seems to me no different than welding anything that requires you to be on or in the thing being welded?

I'm repairing some cracked welds on an old aluminum drift boat.  I need to get inside the boat to reach some of the welds.  It's TIG if that makes any difference.

Make sure you have no exposed skin.  It will be bright.

Don't use the trailer as your grounding point.

Shielding gas can pool in the hull.

Yes depending on the size of the boat you should put a fan at one end pushing the argon out but not disturb your shield gas where your welding, there have been folks die from having their heads stuck in a low point and argon is heavier than air.

I used a mat.  The heat transferred far enough maybe not to the point of getting burnt but uncomfortable.  I put in a shop vac hose on the floor to keep fumes down.  Ground connected to boat.  I tried the ground in various locations but it didn't seem to matter, max distance was maybe 10'.

I'd originally tried to fill one hole with some of that aluminum brazing rod.  It didn't really "wet" to my satisfaction so I ground as much as I could off and welded it.  Made an arcing/burning mess until it was burned away by the plasma.

I did many coupons to try to get proficient but table top is different than awkward positions.  I'm embarrassed to show my work since much of it looks like crap but I did improve a lot.  Most difficult part for me was that the lap (faying?) joints layers would always distort and pull away at the slightest amount of heat.  Nothing to use to clamp or hold that I could come up with easily.

Basically an aluminum drift boat that had 90 degree angle extrusion welded over the bottom to side joint.  That extrusion was worn to knife edge thin.  It was cut along the bottom at 6" intervals to allow it to curve.  I tacked it at first where I could best get it to fit then hammered it to fit and tacked every 6" at the cuts.  Ultimately decided to tack every 3" and finally fill between every fourth set of tacks.

I had probably about 15 pinhole burn throughs that I had to hit from the inside.  Probably will find more when I put it in the water hopefully tomorrow.

Steelhead was closed to fishing from boats in much of WA this year so I didn't lose much.  Decided I'm going to use Wetlander instead of Gluvit for the bottom coat.

ETA: .080" aluminum.  Probably the original metal was 6000 series.  The replacement angle was 5000 series the one that's OK to use 4043 rod on.  4043 seemed to wet better than 5036 (?) not sure if that's the number.  Turned to balance up to more like 40%, seemed to work better with this old aluminum.

What was the frequency setting?  Use a higher frequency on thinner aluminum - something like 120 hz.  In my limited experience, you get a more concentrated arc, and can get moving faster.

Does the boat have any welds in it now? If not, it may not be a weldable aluminum.
Some like 2024 (aka aircraft aluminum) are not weldable. Others like 6061 or 5052 are weldable. 5052 is pretty soft though, but that's just an example.

I set that up near the max of 240 after experimenting on samples.

Yes, welded aluminum boat by a company local called Wooldridge.  It's ca. 1970.  Have had one offer to buy for its vintage appeal by a guide on the Hoh.