Well, you're not alone in haveing a bit of trouble seeing more than just 2 or 3 classes of boats come your way. One thing to keep in mind is the fact that the game requires that you spend at minimum 3 patrols in your starter boat in any given career. After that, the minimum is 4 patrols in a current boat before getting a shot at a different class. On top of that is your performance. I've found that the game is fairly forgiving in this aspect. You don't really need to sink fantastic amounts of tonnage to get to the top of the Sub Aces list, and being there pretty much guarantees you a new boat when one becomes available. As far as availability of the different classes, the TMO manual states what the dates should be for any given port that you want to be attached to. The Balao isn't supposed to become available at Pearl or Midway until the 5th of April in '43. Why this happens so early is a mystery to me. The other issue is that due to hard coding the modders can't do anything about, the game throws refits, rank advancement, and offers of new commands into the same bucket and doles them out in a rather random fashion. This is what Ducimus wrote in the manual about that:
A problem with Rank, Refits, and New commands:
The game seems to throw, rank, refits, and new commands, into the same bucket. If available, the game will always give you a new command or new rank, instead of a refit.
All this being said, I still think something is a bit off, since you and I both got refits to our boats in '42 at a point when, if I was playing the stock game and started with a Tambor or Gar, I always got a Gato. This reliably occured after my 3rd patrol in the starter boat, for which I always chose a Tambor or Gar. The funny thing is that during my current career my boat was refitted with the '42 conning tower after the 3rd patrol, and after the 4th patrol I was promoted and offered the Balao at the same moment. Go figure. You may have seen some discussion of this in other places on the forum, in particular what one has to do in order to get into a new boat at a given time. I was hoping for a Gato myself. I did see a post that Aanker made regarding this, and he asserted that completing objectives, and gaining reknown were key. Tonnage plays some role of course, as tonnage earns you reknown, but objectives completed also seems to be part of the equation. After reading his post, I decided that on any given patrol I would complete 3 primary objectives before returning to port. Prior to that I'd completed only the initial mission, then cruised about building my tonnage score. I made this decision just before getting the refit. In fact I tried to see if I could influence the game to change its decision to give me a refit, loaded up a saved game that had me at Midway refueling on my way back to Pearl, then went back out for another month and added 3 completed objectives to my list. When I got back to Pearl, I still got the refit rather than a new boat. But the very next patrol, I completed 3 objectives, and was promoted and offered the Balao when I got back! Hereafter I will start a new career when this one is played out, and always do extra objectives before returning from patrol and see what happens.
All this being said, I'm not sure its feasible to expect being able to run through as many classes of boats as would be ideal for you. There simply doesn't seem to be enough time in a career to accomplish something like that, since completeing a goal of 3 objectives and getting a decent tonnage score usually takes about 2 months on any given patrol, at least for me, that is.
EDIT: By the way, I have transfered to Freemantle to see if I can get offered a Gato class boat for my next command. The class becomes available there in June of '43. I have 2 more patrols I will need to complete, I think, before that opportunity comes about, as it's been 2 patrols since I turned down the Balao.