Glen,
A 200 amp MiG welder is not adequate for the purpose of welding together the pressure hull of your submarine. The GMAW process commonly known as "MIG" produces very brittle welds when applied to metal thicknesses past the "sheet metal " range ( 10 gauge being the maximum ). A MIG weld might have an excellent appearance but if you cut out some sample "coupons" and subjected them to a "side bend test" you would have an unacceptable failure rate.
Yes, you could TIG (GTAW) weld your whole sub. Tig done properly produces some of the highest quality welds possible, but , it is really slow. For the average person who wants to build his own sub ( and is on a budget ) TIG welding the "roots" and "stick" (SMAW) welding the remaining passes is the way to go. Combination SMAW/GTAW machines are a real bargain and very versatile. The "inverter" types have excellent arc characteristics , are energy efficient and can be plugged into your clothes dryer electrical outlet. Alec has a terrific unit he bought off the internet and to my knowledge is still going strong. He might be willing to share some info on it. I have welded with it and I like it alot !
Use 7018 Low Hydrogen welding rods and 70,000psi tensile strength TIG wire . 3/32" and 1/8" diameter are the recommended sizes. While welding, multiple small passes are much more desireable than one large single pass. Large single passes over heat the metal and sets up stresses , conversely each successive smaller pass tends to anneal the one prior to it. ( Less heat , less stress ) .
If you have not done much welding then by all means take a welding course and practice on some scrap first.
When you do start welding on your hull ; bevel, fit , and do the majority of the welding from the inside of the sub so when it comes time to back grind the roots you can be on the outside. Weld the joint completely ( root, hot pass, filler passes and cap passes )before you do any back grinding.
Daniel Lance