That looks good, the welds look good. Mild tubing is used in everything, cars, buildings, anything construction. If you're going to keep building, try looking for a tubing supply place. I used Roco steel and tubing. I used 16 gauge/.063 less than a buck a foot. I think you'll find mild steel much easier to work with and you get 4-8 welds per connection. The tubing supply place will usually be happy to make the cuts for you. I handed them JonW build cut list and came back in 4 hrs, it was all cut perfect. They charged me $108 with tax. All I had to do was weld it together.
Welding will always be cool!
And I scuba dive every chance I get, scubaSteve