My aluminium house boat is made of 6 mm sheets .
Unfortunately the guy that did all that welding didn't made a best job ( or did what is possible I don't' know).

Anyway welds along whole bottoms leaks, and because boat is flat barge not standing on catamaran hull, that create problem that every water get inside stay there, or need to be pumped.

Can I seal that weld from inside ( at the angle where side walls join bottom ) with some sealant or glue ?

Will the weld will rust in salt water ?

Yes. I agree.

Also, the fact that it leaks means that it is a weak point and there is the possibility that it might rip along the seam from this point if you smack it through a bit of chop, or run over a sand bar.
It's sort of like "tear along the dotted line".

Small house